fix fesetenv(FE_DFL_ENV) on i386

the default fenv was not set up properly, in particular the
tag word that indicates the contents of the x87 registers was
set to 0 (used) instead of 0xffff (empty)

this could cause random crashes after setting the default fenv
because it corrupted the fpu stack and then any float computation
gives NaN result breaking the program logic (usually after a
float to integer conversion).