kirkstone[backport] libcxx: Do not induce -mbranch-protection externally on arm64

[quaresmajose]

Backport https://github.com/kraj/meta-clang/pull/968

On arm64 OE-Core has moved the gcc default configuration done with --enable-standard-branch-protection to be a CFLAGS option which is appended to CC variable, this means that this option can override the package's default to not use it e.g. libunwind where the library has to be built without it.

Fixes https://github.com/kraj/meta-clang/issues/963

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