rx-tdep.c: Use target_read_code instead of target_read_memory.

rx-tdep.c: Use target_read_code instead of target_read_memory.

This change causes the prologue scanner and the frame type scanner in
rx-tdep.c to use target_read_code() instead of target_read_memory().
This change allows these instruction scanners to operate much more
quickly due to the fact that target_read_code() can potentially read
from a cache maintained by GDB.

gdb/ChangeLog:

	* rx-tdep.c (rx_get_opcode_byte): Use target_read_code instead
	of target_read_memory.