[VecLib] Add support for vector fns from Darwin's libsystem.

This patch adds support for Darwin's libsystem math vector functions to
TLI. Darwin's libsystem provides a range of vector functions for libm
functions.

This initial patch only adds the 2 x double and 4 x float versions,
which are available on both X86 and ARM64. On X86, wider vector versions
are supported as well.

Reviewed By: jroelofs

Differential Revision: https://reviews.llvm.org/D101856