fix 64-bit syscall argument passing on or1k

the kernel syscall interface for or1k does not expect 64-bit arguments
to be aligned to "even" register boundaries. this incorrect alignment
broke truncate/ftruncate and as well as a few less-common syscalls.
math: fix x86_64 and x32 asm not to use sahf instruction

Some early x86_64 cpus (released before 2006) did not support sahf/lahf
instructions so they should be avoided (intel manual says they are only
supported if CPUID.80000001H:ECX.LAHF-SAHF[bit 0] = 1).

The workaround simplifies exp2l and expm1l because fucomip can be
used instead of the fucomp;fnstsw;sahf sequence copied from i386.

In fmodl and remainderl sahf is replaced by a simple bit test.
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.