Resolve conflicts between macros in fenv.h and ieeefp.h

This is a follow-up to r321483, which disabled -Wmacro-redefined for
some lib/msun tests.

If an application included both fenv.h and ieeefp.h, several macros such
as fldcw(), fldenv() were defined in both headers, with slightly
different arguments, leading to conflicts.

Fix this by putting all the common macros in the machine-specific
versions of ieeefp.h. Where needed, update the arguments in places
where the macros are invoked.

This also slightly reduces the differences between the amd64 and i386
versions of ieeefp.h.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D15633