libcxx: Do not induce -mbranch-protection externally on arm64

[kraj]

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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[quaresmajose]

We have faced this issue for quiet some time and we disable the -mbranch-protection=standard globally for aarch64 https://github.com/foundriesio/meta-lmp/commit/d4ea7840b0ef58e0cf7ecd518b3be2fd0a166a2a. I could never understand the root cause in my investigations . Thanks for fixing.

[quaresmajose]

This should also be backported to kirkstone.