bazel: Assume default compiler is gcc (#14857)
bazel: Assume default compiler is gcc (#14857)

It is impossible to turn on the "//bazel/gcc_build" config setting on Bazel 3.7.x, see:

   https://github.com/bazelbuild/bazel/issues/12707

GCC 9, which is the default in Ubuntu 20.04, requires the -Wno-maybe-uninitialized for
@com_googlesource_quiche//:quic_core_framer_lib to compile successfully.

Mitigate by assuming the default "compiler" is GCC. Clang is properly recognized by Bazel, so it is not affected by this.

Additional Description: See https://github.com/bazelbuild/bazel/issues/12707
Risk Level: Medium, non-GCC compilers may not implement -Wno-maybe-uninitialized, and may start to fail if not recognized by Bazel.
Testing: Tested compilations with both GCC and clang, and observed that -Wno-maybe-uninitialized is present with GCC but not with clang.
Docs Changes:
Release Notes: Fix compilation with GCC by assuming it is the default compiler.

Signed-off-by: Jarno Rajahalme <0f1ab0ac7dffd9db21aa539af2fd4bb04abc3ad4@covalent.io>
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