fix arm __tlsdesc_dynamic when built as thumb code without __ARM_ARCH>=5

[musl] / src / process / x86_64 / vfork.s

.global vfork
.type vfork,@function
vfork:
        pop %rdx
        mov $58,%eax
        syscall
        push %rdx
        mov %rax,%rdi
        .hidden __syscall_ret
        jmp __syscall_ret