Use gdb_byte * instead of void *

This patch changes the argument type to gdb_byte * in order to align
with the to_xfer_partial interface.

gdb:

2014-01-23  Yao Qi  <yao@codesourcery.com>

	* target.c (raw_memory_xfer_partial): Change argument type
	from void * to gdb_byte *.
	(memory_xfer_partial_1, memory_xfer_partial): Likewise.