[compiler-rt][builtins] Define AT_HWCAP2 for AArch64

Without the definition, build fails on AArch64 with

> error: 'AT_HWCAP2' undeclared (first use in this function);
> did you mean 'AT_HWCAP'?

with old Glibc versions.

Differential Revision: https://reviews.llvm.org/D145494