No toolchain flags are passed to the assembler in cgo mode

[apsaltis-ddog]

rules_go 0.50.1, gazelle 0.43.0.

When attempting to cross-compile https://github.com/DataDog/zstd from macOS (using the gazelle-generated BUILD files) to x86_64 Linux, we got the following cryptic linkage error:

ld.lld: error: /var/folders/2k/slf21zyn0pzdcpbd1mwxwv1h0000gq/T/rules_go_work-1484370210/cgo/github.com/DataDog/zstd/_x46.o: unknown file type
clang: error: linker command failed with exit code 1 (use -v to see invocation)
compilepkg: error running subcommand external/llvm_toolchain/bin/cc_wrapper.sh: exit status 1

After reproducing with --sandbox_debug and running the rules_go builder command with -v --work, we found that this one file was not cross-compiled:

file /.../rules_go_work-1419368796/cgo/github.com/DataDog/zstd/_x46.o
/.../rules_go_work-1419368796/cgo/github.com/DataDog/zstd/_x46.o: Mach-O 64-bit object arm64

And this was because the command run by the builder was not cross-compiled:

SET_A_BUNCH_OF_ENV_VARS= .... \
external/llvm_toolchain/bin/cc_wrapper.sh -c $SANDBOX_DIR/execroot/dd_source/external/com_github_datadog_zstd/huf_decompress_amd64.S -o /.../T/rules_go_work-4033279204/cgo/github.com/DataDog/zstd/_x46.o

This an assembly file, and is the only one built in the repository.

The expected file type would have been at least x86_64 ELF.

I haven't looked into creating a simple repro case, as it would require configuring a C cross-compiler, and disentangling the one we have from our repository is nontrivial.

Regardless, it looks like flags aren't considered in the cgo builder at all. Ideally, rules_go would plumb through the flags for the assemble and/or preprocess-assemble cc toolchain actions, and give an override in go_library/binary/test, although, in my experience, they tend to be the same as the C flags.

Workarounds:

• Patch rules_go to set the sSrcs flags to the same as the C flags in the path mentioned above.
• Patch the gazelle-generated BUILD file to contain a cc_library that corresponds to the cgo sources used by library, and put that in cdeps for the library itself.

A similar ticket is #2612, but it's asking explicitly for flags for the go assembler.