Be more efficient with registers clobbered by syscalls

[josephcsible]

Instead of always pushing and popping the clobbered registers, mark them as clobbers when they're not inputs, and as input/outputs when they are inputs. This will let GCC avoid saving the values in the cases when it knows they won't be needed afterward.

[Cryptogenic]

Sorry for the late merge - looks all good, thanks! Much more elegant.