arm: Add support for interrupt routines to reg_needs_saving_p

arm: Add support for interrupt routines to reg_needs_saving_p

reg_needs_saving_p is only used when dealing with non-interrupt
routines, but it makes sense to extend it to support that context too,
and make arm_compute_save_reg0_reg12_mask use it.

Save only live registers for non-leaf functions, but assume a callee
could clobber any register.

2020-05-15  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/arm.c (reg_needs_saving_p): Add support for interrupt
	routines.
	(arm_compute_save_reg0_reg12_mask): Use reg_needs_saving_p.