[compiler-rt] Implement __clear_cache on FreeBSD/powerpc

dd9173420f06 (Add clear_cache implementation for ppc64. Fix buffer to
meet ppc64 alignment., 2017-07-28), adds an implementation for
__builtin___clear_cache on powerpc64, which was promptly ammended to
also be used with big endian mode in f67036b62c0c (This ppc64 implementation
of clear_cache works for both big and little endian., 2017-08-02)

clang will use this implementation for it's builtin on FreeBSD and result
in an abort() in the cases where 32-bit generation was requested (ex in
macppc or when the big endian powerpc64 build was done with "-m32") and as
reported[1] recently with pcre2, but there is no reason why the same code
couldn't be used in those cases, so use instead the more generic identifier
for the PowerPC architecture.

While at it, update the comment to reflect that POWER8/9 have a 128 byte
wide cache line and so the code could instead use 64 byte windows instead
but that possible optimization has been punted for now.

[1] https://github.com/PhilipHazel/pcre2/issues/92

Reviewed By: jhibbits, #powerpc, MaskRay

Differential Revision: https://reviews.llvm.org/D122640

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