MIPS: Make the extracted stack offset signed in the prologue scanner

MIPS: Make the extracted stack offset signed in the prologue scanner

Make the extracted stack offset signed in the standard MIPS prologue
scanner, to simplify handling and make sure register offsets are correct
in all cases, especially where $fp equals the virtual frame pointer (old
GCC frames) and therefore offsets to save slots are negative.

	* mips-tdep.c (mips32_scan_prologue): Make the extracted stack
	offset signed.