arm: Use POP {pc} to return when returning [PR118089]

When generating thumb2 code,
	LDM SP!, {PC}
is a two-byte instruction, whereas
	LDR PC, [SP], #4
is needs 4 bytes.  When optimizing for size, or when there's no obvious
performance benefit prefer the former.

gcc/ChangeLog:

	PR target/118089
	* config/arm/arm.cc (thumb2_expand_return): Use LDM SP!, {PC}
	when optimizing for size, or when there's no performance benefit over
	LDR PC, [SP], #4.
	(arm_expand_epilogue): Likewise.