fix fenv exception functions to mask their argument

fesetround.c is a wrapper to do the arch independent argument
check (on archs where rounding mode is not stored in 2 bits
__fesetround still has to check its arguments)

on powerpc fe*except functions do not accept the extra invalid
flags of its fpscr register

the useless FENV_ACCESS pragma was removed from feupdateenv

fenv: return FE_TONEAREST in dummy fegetround

floating point environment, untested

at present the i386 code does not support sse floating point, which is
not part of the standard i386 abi. while it may be desirable to
support it later, doing so will reduce performance and require some
tricks to probe if sse support is present.

this first commit is i386-only, but it should be trivial to port the
asm to x86_64.