Make _LIBUNWIND_SUPPORT_FRAME_APIS a build-time option

Previously this was based on target architecture, but
that makes very little sense--frame API availability is generally
for libgcc compatibility and that is dependent on runtime
needs rather than target architecture.

Default this to on, so as not to remove the apis from
environments that already have them.

The functions this macro protects are stubs for libgcc-compatibility.
Today, libunwind as a drop-in replacement for libgcc_eh links on x86,
x86_64, and powerpc, but not aarch64, which doesn't really make
sense. As there is nothing architecture specific about these, they
should be provided everywhere or nowhere.

The target-specific protection goes all the way back to the original
code contribution in 312fcd0e from
2013, so the original reason is lost to history, and probably not
relevant today.

Differential Revision: https://reviews.llvm.org/D158011