Use 16-bit integer type for rl78 register pairs.

This patch changes rl78-tdep.c so that a 16-bit type is used for
register pairs instead of a pointer type as was previously the case.
This will cause these register pairs to be displayed as integers
instead of as a data address with a 0xf0000 ORed in.

E.g. registers ax, bc, de, and hl might now be displayed like this:

    (gdb) info registers ax bc de hl
    ax             0x6      6
    bc             0x0      0
    de             0x10c3   4291
    hl             0x108d   423

Whereas, before, they were displayed as follows:

    (gdb) info registers ax bc de hl
    ax             0xf0006  0xf0006
    bc             0xf0000  0xf0000
    de             0xf10c3  0xf10c3
    hl             0xf108d  0xf108d

These pairs are 16 bit quantities and should be displayed as such.
This change also affects the way that the banked register pairs are
displayed.  Within GDB, the banked register pairs are named bank0_rp0,
bank0_rp1, .., bank3_rp2, bank3_rp3.

However, these register pairs need to be used as addresses in DWARF
expressions.  I have added 16 pseudo registers corresponding to banked
register pairs.  These new pseudo registers are all hidden from the
user and have a pointer type.  Values from these registers are
intended to be used in DWARF expressions.  Therefore,
rl78_dwarf_reg_to_regnum() has been adjusted to return these new
pseudo register numbers.

I had a much simpler patch which only changed the types, but it showed
a number of regressions due to integer values from the banked register
pairs being used as part of an address expression.  This current patch
shows no regressions and now displays values of register pairs
correctly.

	* rl78-tdep.c ( RL78_BANK0_RP0_PTR_REGNUM, RL78_BANK0_RP1_PTR_REGNUM)
	(RL78_BANK0_RP2_PTR_REGNUM, RL78_BANK0_RP3_PTR_REGNUM)
	(RL78_BANK1_RP0_PTR_REGNUM, RL78_BANK1_RP1_PTR_REGNUM)
	(RL78_BANK1_RP2_PTR_REGNUM, RL78_BANK1_RP3_PTR_REGNUM)
	(RL78_BANK2_RP0_PTR_REGNUM, RL78_BANK2_RP1_PTR_REGNUM)
	(RL78_BANK2_RP2_PTR_REGNUM, RL78_BANK2_RP3_PTR_REGNUM)
	(RL78_BANK3_RP0_PTR_REGNUM, RL78_BANK3_RP1_PTR_REGNUM)
	(RL78_BANK3_RP2_PTR_REGNUM, RL78_BANK3_RP3_PTR_REGNUM):
	New constants.
	(rl78_register_type): Use a data pointer type for SP and
	new pseudo registers mentioned above.  Use a 16 bit integer
	type for all other register pairs.
	(rl78_register_name, rl78_g10_register_name): Update for
	new pseudo registers.
	(rl78_pseudo_register_read): Likewise.
	(rl78_pseudo_register_write): Likewise.
	(rl78_dwarf_reg_to_regnum): Return register numbers representing
	to the newly added pseudo registers.