arm: fix UB when compiling thumb2 with PIC [PR100236]

arm: fix UB when compiling thumb2 with PIC [PR100236]

arm_compute_save_core_reg_mask contains UB in that the saved PIC
register number is used to create a bit mask. However, for some target
options this register is undefined and we end up with a shift of ~0.

On native compilations this is benign since the shift will still be
large enough to move the bit outside of the range of the mask, but if
cross compiling from a system that truncates out-of-range shifts to
zero (or worse, raises a trap for such values) we'll get potentially
wrong code (or a fault).

gcc:
	PR target/100236
	* config/arm/arm.c (THUMB2_WORK_REGS): Check PIC_OFFSET_TABLE_REGNUM
	is valid before including it in the mask.