freebsd32: rename 32-bit compat pads to _pad

Some 32-bit architectures pass 64-bit values in aligned
register pairs (a0,a1), (a2,a3) etc. In freebsd32 we add these pads
explicitly from compat code. We also sometimes add pads in the default
ABI. Differentiate the two by making the freebsd32 ones int _pad.
In a future commit the 32-bit ones will be automatically generated.

Reviewed by: kevans