[libc][cmake] split fputil into individual targets

The libc.src.__support.FPUtil.fputil target encompassed many unrelated
files, and provided a lot of hidden dependencies. This patch splits out
all of these files into component parts and cleans up the cmake files
that used them. It does not touch any source files for simplicity, but
there may be changes made to them in future patches.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D132980