test: readd TMPDIR to LD_LIBRARY_PATH for run-make

It was accidentally removed in #15006 and that somehow got past the
build bots, causing `src/test/run-make/c-dynamic-dylib` to fail on at
least my linux system.

This resolves #15103 (thanks to @alexcrichton!).
Allow trailing comma in `concat!`

(And in other extensions implemented with `get_exprs_from_tts` function).
librustc: Remove outdated reference to `~` and `@`

Fix #15052
auto merge of #15071 : tomjakubowski/rust/fix-15052, r=alexcrichton

Fix #15052
auto merge of #15114 : ben0x539/rust/run-make-libpath, r=alexcrichton

It was accidentally removed in #15006 and that somehow got past the
build bots, causing `src/test/run-make/c-dynamic-dylib` to fail on at
least my linux system.

This resolves #15103 (thanks to @alexcrichton!).
auto merge of #15118 : stepancheg/rust/concat, r=alexcrichton

(And in other extensions implemented with `get_exprs_from_tts` function).