[compiler-rt] Support FreeBSD standalone (boot) environment

FreeBSD uses -Ddouble=jagged-little-pill -Dfloat=floaty-mcfloatface to
poison uses of floating point in its standalone environment. It also
deprecates machine/limits.h in favour of sys/limits.h and does not even
provide the former on newer architectures.

This is a cleaner reimplementation of equivalent patches in FreeBSD's
vendored copy of compiler-rt.

Reviewed By: dim

Differential Revision: https://reviews.llvm.org/D95264