math: use fnstsw consistently instead of fstsw in x87 asm

fnstsw does not wait for pending unmasked x87 floating-point exceptions
and it is the same as fstsw when all exceptions are masked which is the
only environment libc supports.

math: add drem and dremf weak aliases to i386 remainder asm

weak_alias was only in the c code, so drem was missing on platforms
where remainder is implemented in asm.

assembly optimizations for fmod/remainder functions