mips: fix void*/gdb_byte* compilation errors

mips: fix void*/gdb_byte* compilation errors

The error in mips64_linux_get_longjmp_target is fixed by changing "buf"
to be a gdb_byte*, as usual.  supply_32bit_reg and mips64_fill_gregset
do some more complicated things however, so it's safer just to add the
explicit cast and avoid changing the code too much.

gdb/ChangeLog:

	* mips-linux-tdep.c (mips64_linux_get_longjmp_target): Change type of
	buf to gdb_byte*.
	(supply_32bit_reg): Add cast.
	(mips64_fill_gregset): Likewise.