Changes needed for mips suport have been added.
Code for openlibm_fenv_mips.h, mips_fpmath.h and fenv.c
was taken from https://github.com/freebsd/freebsd
While there, also modify the install target. We should make sure to
install all openlibm*.h headers. There is still some work to be done:
openlibm_fenv_*.h still depends on some additional bits. I'd propose
that we eventually create an include/openlibm_cdefs.h that contains all
of the macros we need.
