Make FPUD_LOAD/FPUD_STORE portable to x86_64
Make FPUD_ARITH1/2/3 portable to x86_64
Replace REMINDER by REMAINDER
Make FPUD_REMAINDER/COMPARE/EXAMINE portable to x86_64
Make FPUD_LOAD_CONST portable to x86_64
Make FPUD_TRIG/SINCOS portable to x86_64
Make FPUD_XTRACT portable to x86_64
Make FPUD_WITH_POP portable to x86_64
Make FPUD_PTAN portable to x86_64
Make FPUD_FYL2X portable to x86_64
Enable fpu-x86 on x86_64 too
Make exc_mask/fclex a bit more consistent
Allow gcc to use ax for fnstsw where possible
Remove WEAK_EXCEPTIONS variant of ARITH3 (for fdiv, fdivr)

ARITH3 was introduced in r2425 as a variant of ARITH1 without a
WEAK_EXCEPTIONS variant. In r2644 a WEAK_EXCEPTIONS variant was
seemingly accidentally added. This restores the original behaviour.