Repository: rust
Issue: wasm target not honoring linker set at bootstrap time (1.42.0 and maybe earlier)
Labels: A-linkage,T-compiler,O-wasm,C-bug

rust for gentoo linux (the one we ship in official repositories)

here's how we configure wasm target:

```toml
[target.wasm32-unknown-unknown]
linker = "lld"
```

but it still looks for rust-lld for that target by default.

```shel-session
cargo build --release --target=wasm32-unknown-unknown --verbose
   Compiling wasm-test v0.1.0 (/tmp/wasm-test)
     Running `rustc --crate-name wasm_test src/lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type cdylib --emit=dep-info,link -C opt-level=3 -C metadata=7a89689720da237e --out-dir /tmp/wasm-test/target/wasm32-unknown-unknown/release/deps --target wasm32-unknown-unknown -L dependency=/tmp/wasm-test/target/wasm32-unknown-unknown/release/deps -L dependency=/tmp/wasm-test/target/release/deps`
error: linker `rust-lld` not found
  |
  = note: No such file or directory (os error 2)

error: aborting due to previous error

error: could not compile `wasm-test`.

Caused by:
  process didn't exit successfully: `rustc --crate-name wasm_test src/lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type cdylib --emit=dep-info,link -C opt-level=3 -C metadata=7a89689720da237e --out-dir /tmp/wasm-test/target/wasm32-unknown-unknown/release/deps --target wasm32-unknown-unknown -L dependency=/tmp/wasm-test/target/wasm32-unknown-unknown/release/deps -L dependency=/tmp/wasm-test/target/release/deps` (exit code: 1)
```


not sure when it started to happen, but it was working as expected before.
`rustc -Z unstable-options --target=wasm32-unknown-unknown --print target-spec-json`
```json
{
  "arch": "wasm32",
  "data-layout": "e-m:e-p:32:32-i64:64-n32:64-S128",
  "default-hidden-visibility": true,
  "dll-prefix": "",
  "dll-suffix": ".wasm",
  "dynamic-linking": true,
  "emit-debug-gdb-scripts": false,
  "env": "",
  "exe-suffix": ".wasm",
  "executables": true,
  "has-elf-tls": true,
  "is-builtin": true,
  "limit-rdylib-exports": false,
  "linker": "rust-lld",
  "linker-flavor": "wasm-ld",
  "lld-flavor": "wasm",
  "llvm-target": "wasm32-unknown-unknown",
  "max-atomic-width": 64,
  "only-cdylib": true,
  "os": "unknown",
  "panic-strategy": "abort",
  "pre-link-args": {
    "gcc": [
      "-Wl,--no-threads",
      "-Wl,-z",
      "-Wl,stack-size=1048576",
      "-Wl,--stack-first",
      "-Wl,--allow-undefined",
      "-Wl,--fatal-warnings",
      "-Wl,--no-demangle",
      "-Wl,--export-dynamic",
      "--target=wasm32-unknown-unknown",
      "-nostdlib",
      "-Wl,--no-entry"
    ],
    "wasm-ld": [
      "--no-threads",
      "-z",
      "stack-size=1048576",
      "--stack-first",
      "--allow-undefined",
      "--fatal-warnings",
      "--no-demangle",
      "--export-dynamic",
      "--no-entry"
    ]
  },
  "relocation-model": "static",
  "simd-types-indirect": false,
  "singlethread": true,
  "target-c-int-width": "32",
  "target-endian": "little",
  "target-pointer-width": "32",
  "tls-model": "local-exec",
  "vendor": "unknown"
}
```

for some reason target spec still lists `rust-lld` as preferred linker

Yes, there are ways to mitigate that, like
`CARGO_TARGET_WASM32_UNKNOWN_UNKNOWN_LINKER=lld`
or 
`RUSTFLAGS="-C linker=/usr/bin/lld`

lld is system installed and provides the following files

```
/usr/bin/lld
/usr/bin/wasm-ld -> lld
/usr/bin/ld64.lld -> lld
/usr/bin/ld.lld -> lld
/usr/bin/lld-link -> lld
```

if we build using bundled llvm and lld, everything works as expected ofc.

full config.toml, happens on x86_64 as well, so powerpc being primay is irrelevant.
```toml
[llvm]
optimize = true
release-debuginfo = false
assertions = false
targets = "WebAssembly;PowerPC;AMDGPU"
experimental-targets = ""
link-shared = true
[build]
build = "powerpc64le-unknown-linux-gnu"
host = ["powerpc64le-unknown-linux-gnu"]
target = ["powerpc64le-unknown-linux-gnu","wasm32-unknown-unknown"]
cargo = "/usr/bin/cargo"
rustc = "/usr/bin/rustc"
docs = false
compiler-docs = false
submodules = false
python = "python3.6"
locked-deps = true
vendor = true
extended = true
tools = ["rustfmt","rls","analysis","src","miri","clippy","cargo",]
verbose = 2
[install]
prefix = "/usr"
libdir = "lib"
docdir = "share/doc/rust-1.42.0-r1"
mandir = "share/man"
[rust]
optimize = true
debug = false
debug-assertions = false
default-linker = "powerpc64le-unknown-linux-gnu-gcc"
parallel-compiler = true
channel = "nightly"
rpath = false
lld = false
backtrace-on-ice = true
[dist]
src-tarball = false
[target.powerpc64le-unknown-linux-gnu]
cc = "powerpc64le-unknown-linux-gnu-gcc"
cxx = "powerpc64le-unknown-linux-gnu-g++"
linker = "powerpc64le-unknown-linux-gnu-gcc"
ar = "powerpc64le-unknown-linux-gnu-ar"
llvm-config = "/usr/lib/llvm/10/bin/llvm-config"
[target.wasm32-unknown-unknown]
linker = "lld"
```