Fix LTO UI tests

[antoyo]

In this PR, I ignored a few tests that were failing when the sysroot was compiled with LTO.

Those tests fail because a symbol is not found in some proc-macro.

• rustc_codegen_gcc/build_sysroot/Cargo.toml

  Line 22 in 4e41a8a

  #lto = "fat" # TODO(antoyo): re-enable when the failing LTO tests regarding proc-macros are fixed.

• rustc_codegen_gcc/.github/workflows/release.yml

  Line 98 in 4e41a8a

  # FIXME(antoyo): we cannot enable LTO for stdarch tests currently because of some failing LTO tests using proc-macros.

• rustc_codegen_gcc/.github/workflows/release.yml

  Line 109 in 4e41a8a

  run: cat failing-lto-tests.txt >> failing-ui-tests.txt

• rustc_codegen_gcc/.github/workflows/stdarch.yml

  Line 139 in 4e41a8a

  # FIXME: these tests fail when the sysroot is compiled with LTO because of a missing symbol in proc-macro.