gnullvm: link libunwind statically by default

[mati865]

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Previously shared library was used meaning that programs and libraries couldn't be loaded if libunwind.dll was missing from the PATH. Using Wine (on Linux) because it better shows the problem (and is more convenient):

❯ cargo new hello &> /dev/null

❯ cargo rustc --target x86_64-pc-windows-gnullvm &> /dev/null

❯ wine target/x86_64-pc-windows-gnullvm/debug/hello.exe
0024:err:module:import_dll Library libunwind.dll (which is needed by L"Z:\\tmp\\hello\\target\\x86_64-pc-windows-gnullvm\\debug\\hello.exe") not found
0024:err:module:loader_init Importing dlls for L"Z:\\tmp\\hello\\target\\x86_64-pc-windows-gnullvm\\debug\\hello.exe" failed, status c0000135

❯ llvm-readobj --coff-imports target/x86_64-pc-windows-gnullvm/debug/hello.exe
...
Import {
  Name: libunwind.dll
  ImportLookupTableRVA: 0x3D308
  ImportAddressTableRVA: 0x3D678
  Symbol: _GCC_specific_handler (0)
  Symbol: _Unwind_DeleteException (0)
  Symbol: _Unwind_GetDataRelBase (0)
  Symbol: _Unwind_GetIPInfo (0)
  Symbol: _Unwind_GetLanguageSpecificData (0)
  Symbol: _Unwind_GetRegionStart (0)
  Symbol: _Unwind_GetTextRelBase (0)
  Symbol: _Unwind_RaiseException (0)
  Symbol: _Unwind_Resume (0)
  Symbol: _Unwind_SetGR (0)
  Symbol: _Unwind_SetIP (0)
}
...

Optionally libunwind could be linked statically via +crt-static:

❯ cargo rustc --target x86_64-pc-windows-gnullvm -- -C target-feature=+crt-static &> /dev/null

❯ wine target/x86_64-pc-windows-gnullvm/debug/hello.exe
Hello, world!

❯ llvm-readobj --coff-imports target/x86_64-pc-windows-gnullvm/debug/hello.exe | rg 'libunwind.dll' || echo "doesn't depend on shared libunwind"
doesn't depend on shared libunwind

This PR turns that the other way, libunwind will be now linked statically by default and could be linked dynamically with -crt-static:

❯ wine target/x86_64-pc-windows-gnullvm/debug/hello.exe
Hello, world!

❯ llvm-readobj --coff-imports target/x86_64-pc-windows-gnullvm/debug/hello.exe | rg 'libunwind.dll' || echo "doesn't depend on shared libunwind"
doesn't depend on shared libunwind

❯ cargo rustc --target x86_64-pc-windows-gnullvm -q -- -C target-feature=-crt-static &> /dev/null

❯ wine target/x86_64-pc-windows-gnullvm/debug/hello.exe
0024:err:module:import_dll Library libunwind.dll (which is needed by L"Z:\\tmp\\hello\\target\\x86_64-pc-windows-gnullvm\\debug\\hello.exe") not found
0024:err:module:loader_init Importing dlls for L"Z:\\tmp\\hello\\target\\x86_64-pc-windows-gnullvm\\debug\\hello.exe" failed, status c0000135

❯ llvm-readobj --coff-imports target/x86_64-pc-windows-gnullvm/debug/hello.exe | rg 'libunwind.dll' || echo "doesn't depend on shared libunwind"
  Name: libunwind.dll

This is how I intended it to work previously, but couldn't manage to get it working 🤷🏻
Related PRs: #121794 #122003

[mati865]

@bors try jobs=dist-x86_64-llvm-mingw

[rust-bors]

💔 Test for be73c40 failed: CI. Failed job:

• try - dist-x86_64-llvm-mingw (web logs, enhanced plaintext logs)

[mati865]

@bors try jobs=dist-x86_64-llvm-mingw

[rust-bors]

💔 Test for e559b52 failed: CI. Failed job:

• try - dist-x86_64-llvm-mingw (web logs, enhanced plaintext logs)

[mati865]

This is how I intended it to work previously, but couldn't manage to get it working 🤷🏻

So, that must be why...

ld.lld: error: undefined symbol: _Unwind_Resume
#159782 (comment)

[mati865]

@bors try jobs=dist-x86_64-llvm-mingw,dist-i686-llvm-mingw

[rust-bors]

💔 Test for ae7481e failed: CI. Failed job:

• Calculate job matrix (web logs, enhanced plaintext logs)

[mati865]

@bors try jobs=dist-*-llvm-mingw

[rust-bors]

💔 Test for 9539b53 failed: CI. Failed jobs:

• try - dist-aarch64-llvm-mingw (web logs, enhanced plaintext logs)
• try - dist-x86_64-llvm-mingw (web logs, enhanced plaintext logs)

[mati865]

@bors try jobs=dist-*-llvm-mingw

[rust-bors]

☀️ Try build successful (CI)
Build commit: cf04f4c (cf04f4cb78db05a8f7f4ef0a5c2dc58b8eabbad8)
Base parent: 1b29cfd (1b29cfdbee8ee37333f4b24afb936350eb57e705)