Ignore warnings in specific musl system headers

Musl is not warnings-clean, so we need to ignore a few warnings from its
headers. We previously ignored these warnings by including the musl
headers with -isystem, but for the SDK sysroot this would ignore
warnings in the entire sysroot, which is not desirable.

Adds a clang pragma to ignore warnings from system headers from only the
necessary headers.

Test: build.py qemu-generic-arm64-test-debug
Bug: 169448829
Change-Id: I2536176f4929d960a61f5fc97a423aa5a411d975
3 files changed