Move mut spec to argument.

The more I think about this, the more I think this is more correct.
Considering that the argument is owned, by the function, it seems
more correct to just recognize that in the signature. It is not a
breaking change since the arg is passed by value since it's a primitive
type, so I think this is better.

In principle, I would prefer to merge this with the commit that moves
the asm code into the crate directly instead of this separate commit.

Author: wt

cortex-m/src/asm.rs

@@ -239,8 +239,7 @@ pub unsafe fn bx_ns(addr: u32) {
 /// This method is used by cortex-m-semihosting to provide semihosting syscalls.
 #[inline]
 #[cortex_m_macros::asm_wrapper(cortex_m)]
-pub unsafe fn semihosting_syscall(nr: u32, arg: u32) -> u32 {
-    let mut nr = nr;
+pub unsafe fn semihosting_syscall(mut nr: u32, arg: u32) -> u32 {
     unsafe {
         asm!("bkpt #0xab", inout("r0") nr, in("r1") arg, options(nomem, nostack, preserves_flags))
     };