Support gdbarch_convert_register_p targets in address_from_register

Since the last change to address_from_register, it no longer supports
targets that require a special conversion (gdbarch_convert_register_p)
for plain pointer type; I had assumed no target does so.

This turned out to be incorrect: MIPS64 n32 big-endian needs such a
conversion in order to properly sign-extend pointer values.

This patch fixes this regression by handling targets that need a
special conversion in address_from_register as well.

gdb/ChangeLog:

	* findvar.c (address_from_register): Handle targets requiring
	a special conversion routine even for plain pointer types