Ensure that compiler environment is passed to gyp

this only affects libuv
Copy GYP environment variables on iOS
Add cargo.vim compiler file.
Link libgcc statically on Win64.
Allow linking it statically on Win32 with an override.
Use locale 'C' for running tests. Closes #17423
collapse setting and exporting RUST_BENCH into one line
Fix regression and overflow bug for rationals.
auto merge of #17248 : jbcrail/rust/fix-rational-rounding, r=alexcrichton

When I fixed the previous issue with rational rounding, I had introduced a regression. There was also an overflow bug introduced for fixed-precision rationals. This patch corrects both bugs.
auto merge of #17449 : mcoffin/rust/master, r=alexcrichton

Right now, libuv will **always** be built for the host system (at least when building on OSX) because the information about the cross compiler is never actually passed to GYP. I don't know how anybody has been managing to build cross compilers with this.

Note that, at least on OSX, there is a bug in GYP that will send clang flags to non-clang compilers and it will still attempt to use Xcode's libtool, so this doesn't completely fix the problem of cross-compiling on an OSX host, but it's a start.
auto merge of #17459 : dradtke/rust/master, r=brson

This PR adds a new Vim compiler file specifically for use with Cargo. It passes all arguments through, so commands like `:make build`, `:make clean`, and `:make run` all work as expected.

It also adds a quickfix autocommand for fixing the paths before populating the error list. `cargo build` reports errors with file paths that are relative to Cargo.toml, so if you're further down in the project tree, then trying to open the error will result in a blank buffer because Vim treats that path as relative to the working directory instead. With this fix, the paths work properly no matter where you are in the project.
auto merge of #17463 : oskchaitanya/rust/master, r=alexcrichton

Setting LC_ALL to C helps keep gdb's output consistent ('print' gives us expected output). This fixes #17423. I do not have access to a windows/mac machines to test this. I've only tested it on an x86_64 linux box.
auto merge of #17471 : vadimcn/rust/link-libgcc, r=alexcrichton

Closes #17271
Closes #15420