Add LLD flags for MinGW

[mati865]

Tested locally and this now works:

• RUSTFLAGS="-Zlink-self-contained=yes -Clinker=rust-lld" cargo b
• RUSTFLAGS="-Zlink-self-contained=no -Clinker=rust-lld -Zpre-link-arg=-Ld:/msys64/mingw64/x86_64-w64-mingw32/lib -Zpre-link-arg=-Ld:/msys64/mingw64/lib/gcc/x86_64-w64-mingw32/10.2.0 -Zpre-link-arg=crt2.o" cargo b

This is "harmless" part of the changes to make possible linking with bare LLD with windows-gnu target. More debatable changes should follow in next PRs soon.

[rust-highfive]

r? @matthewjasper

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[petrochenkov]

r? @petrochenkov

[mati865]

FTR afb3aeb is preliminary commit to enable LLD by default with self-contained mode but it depends on adding heuristic to crt_objects_fallback which in turn depends on rust-lang/compiler-team#343

[petrochenkov]

So, we don't generally use linker directly for non bare-metal targets given the state of #11937, but using it in self-contained mode only and avoiding shipping ancient gcc is a good motivation to make an exception, especially if the replacement is fresh rust-lld that we are already shipping.

[petrochenkov]

Is lld generally usable for mingw targets yet?
Issues like #68647 still exist, so it will likely fail to link a random sizable project.

[petrochenkov]

@bors r+

[bors]

📌 Commit c75f7f9 has been approved by petrochenkov

[bors]

⌛ Testing commit c75f7f9 with merge e019be06c2a36f6bbb0456fb9dce423d797cc859...

[petrochenkov]

@bors r-
Forgot to check the options not duplicated for LLD.
--nxcompat is missing (in windows_gnu_base.rs).

[mati865]

@petrochenkov #68647 is fixed in MSYS2 MinGW LLD packages and in upcoming LLVM 11 which Rust is hopefully going to use soon: #73526

Is lld generally usable for mingw targets yet?

At least when it comes to 64-bit LLD is generally usable for C/C++ code since few releases, for Rust since #72049 (but requires defining WINAPI_NO_BUNDLED_LIBRARIES until retep998/winapi-rs#935 is resolved).
I've seen 32-bit LLD 10 to crash when linking LLVM with it.

So, we don't generally use linker directly for non bare-metal targets given the state of #11937

TBH calling linker directly is basically the same as calling $CC with -nostdlib which Rust did for long time with MinGW. To have it working without issues Rust would have to get paths by calling gcc --print-file-name crt2.o and gcc --print-file-name libgcc.a or similar.
I do agree it's easier to just pass -fuse-ld=lld.

--nxcompat is missing (in windows_gnu_base.rs).

LLD silently ignores --nxcompat since it's always enabled.

[petrochenkov]

@bors r+

[bors]

📌 Commit c75f7f9 has been approved by petrochenkov

[bors]

⌛ Testing commit c75f7f9 with merge 5addb13...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: petrochenkov
Pushing 5addb13 to master...