Use clang by default for mips builds
Use clang by default for mips builds

This matches the current behavior or all the other
linux builds.

There seems to be single warning (from the
GNU assembler) in the entire build, but this also
occurs with gcc:
Warning: tried to set unrecognized symbol: oddspreg

TEST=GYP_DEFINES="target_arch=mipsel" GYP_CROSSCOMPILE=1 build/gyp_chromium && ninja -C out/Release

Review URL: https://codereview.chromium.org/1260803002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#340991}

ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend