Extend rl78 prologue analyzer to recognize more complicated prologues

Extend rl78 prologue analyzer to recognize more complicated prologues

This patch extends the rl78 prologue analyzer so that it can recognize
this kind of prologue:

   0x119f <main>:       movw    ax, sp
   0x11a1 <main+2>:     subw    ax, #0x1fa6
   0x11a4 <main+5>:     movw    sp, ax

The test case for gdb.base/miscexprs.exp is now compiled to generate
that sequence instead of a much longer and more inefficient sequence.

gdb/ChangeLog:

	* rl78-tdep.c (RL78_SP_ADDR): Define.
	(opc_reg_to_gdb_regnum): New static function.
	(rl78_analyze_prologue): Recognize instructions forming slightly
	more interesting prologues.