[compiler-rt] [safestack] Enable for aarch64

This patch enables the safestack for aarch64. The frontend already have
it enabled on all supported architectures and no adjustment is required
in llvm.

The compiler-rt adjustments are basically add on the cmake configuration
to enable the tests and fix the pagesize debug check by getting its
value at runtime (since aarch64 has multiple pagesize depending of
kernel configuration).

llvm-svn: 255345