instead of just detecting Bazel's default "compiler" string.

When Bazel auto-configures GCC, it sets the compiler string to
"compiler", probably for backwards compatibility. Some users manually
set the string to "gcc".

This should address the backwards compatibility issues described in
https://github.com/bazelbuild/bazel/issues/12707 and hopefully fix
https://github.com/abseil/abseil-cpp/issues/1263

PiperOrigin-RevId: 473069817
Change-Id: I8a24721f63f9d61447b22b3b05b06a9dde7d34d8