Restrict matching add/sub sp, #imm

Restrict matching add/sub sp, #imm

Currently, GDB matches both add/sub sp, #imm in prologue and epilogue,
which is not very precise.  On the instruction level, the immediate
number in both instruction can't be negative, so 'sub sp, #imm' only
appears in prologue while 'add sp, #imm' only appears in epilogue.
Note that on assembly level, we can write 'add sp, -8', but gas will
translate to 'sub sp, 8' instruction.

This patch is to only match 'sub sp, #imm' in prologue and match
'add sp, #immm' in epilogue.  It paves the way for the following
patch.

gdb:

2014-07-11  Yao Qi  <yao@codesourcery.com>

	* arm-tdep.c (thumb_analyze_prologue): Don't match instruction
	'add sp, #imm'.
	(thumb_in_function_epilogue_p): Don't match 'sub sp, #imm'.