ARM: Do not use FP reg when on AAPCS

ARM: Do not use FP reg when on AAPCS

GDB tries to dereference the frame pointer in arm_scan_prologue as a
last resort to create frame information.
However, the more recent AAPCS ABI does not make use of a frame pointer.

This patch checks whether the specified arm_abi is AAPCS before
dereferencing the "frame pointer". If so, just return as efforts to use
it for restoring frame information won't work.

gdb/ChangeLog
2018-11-06  Marius Muench  <marius.muench@eurecom.fr>

	* arm-tdep.c (arm_scan_prologue): Don't dereference FP reg
	when on AAPCS.