[compiler-rt] Include system headers before optionally defining HWCAP macros

In https://reviews.llvm.org/D141285 the optional definitions of `HWCAP`
macros were moved to before their usage. However, they were also moved
to before the inclusion of system headers which can optionally define
them. If any of those system headers then actually defined any of the
`HWCAP` macros, it would result in a redefinition error.

Move the system header includes to just before the optional definitions,
to avoid this problem.

Reviewed By: ilinpv

Differential Revision: https://reviews.llvm.org/D148819