Switch From Clang To GCC To Fix Code Coverage Issues

[garethellis0]

Description of the task

In #985, we setup our toolchain to use arm-none-eabi-gcc for arm targets, and clang for x86_64 targets. Unfortunately, bazel has issues getting proper code coverage from clang, and so we have a somewhat hacky script to manually convert the coverage outputs from clang into a format that CodeCov can understand. As such we would like to switch back to gcc, which bazel properly supports in terms of reading coverage (at least for some versions of gcc). We should use the newest version of gcc possible.

Relevant Bazel Issues

bazelbuild/bazel#9406
bazelbuild/bazel#8217
bazelbuild/bazel#7719

Acceptance criteria

• Download gcc instead of clang via the WORKSPACE file the associated external build file
• Change the wrapper scripts in cc_toolchain/wrappers for clang to gcc
• Implement a gcc toolchain configuration in cc_toolchain_config.bzl and remove the clang one
• Change everything in cc_toolchain/BUILD from clang to gcc
• Remove the convert_profdata_to_lcov script, and make sure codecov still works (will probably need to add back the -s bazel-testlogs flag to the codecov command in .travis.yml)

Blocked By

#1124

[jonathanlew]

Will look into this after #1129 gets merged

[MathewMacDougall]

Was this not resolved by #1184 ?

[jonathanlew]

Was this not resolved by #1184 ?

Yes