Revise signal mapping function in GDB interface for RX sim.

	sim/rx/gdb-if.c had a function named `rx_signal_to_host'.  This
	function mapped signal numbers used by the BSP to host signal
	numbers which, at one time a while back, were used by GDB
	as target signal numbers.  This change updates the signal
	numbers returned to be those names prefixed with "GDB_SIGNAL_"
	as defined in include/gdb/signals.h.

	It also changes the name of the function somewhat to better
	match what the function currently does.  I noticed that this
	function is not static - and there's no reason for it not
	to be - so I made it static too.

	* gdb-if.c (rx_signal_to_host):  Rename to
	`rx_signal_to_gdb_signal'.  Make static.  Update all callers
	to use new name.  Use signal names from include/gdb/signals.h.