siglongjmp(): Preserve floating point exception flags on i386 and amd64.

Per POSIX, siglongjmp() shall be equivalent to longjmp() except that it must
match sigsetjmp() instead of setjmp() and except for the effect on the
signal mask. Therefore, it should preserve the floating point exception
flags.

This was fixed for longjmp() and _longjmp() in r180080 and r180081 for amd64
and i386 respectively.