arm: smh: Allow semihosting trap calls to be inlined

Currently our semihosting trap function is somewhat fragile: we rely
on the current compiler behaviour to assign the second inline assembly
argument to the next free register (r1/x1), which happens to be the
"addr" argument to the smh_trap() function (per the calling convention).
I guess this is also the reason for the noinline attribute.

Make it explicit what we want: the "addr" argument needs to go into r1,
so we add another register variable. This allows to drop the "noinline"
attribute, so now the compiler beautifully inlines just the trap
instruction directly into the calling function.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>