math: use fnstsw consistently instead of fstsw in x87 asm

[musl] / src / math / x86_64 / fmodl.s

.global fmodl
.type fmodl,@function
fmodl:
        fldt 24(%rsp)
        fldt 8(%rsp)
1:      fprem
        fnstsw %ax
        testb $4,%ah
        jnz 1b
        fstp %st(1)
        ret