arm: Fix up gcc.target/arm/lto/pr96939_* FAIL

arm: Fix up gcc.target/arm/lto/pr96939_* FAIL

The following patch on top of the
https://gcc.gnu.org/pipermail/gcc-patches/2020-September/553801.html
patch fixes the gcc.target/arm/lto/pr96939_* test in certain ARM
configurations.
As said in the above mentioned patch, the generic code takes care of
saving/restoring TargetVariables or Target Save options, so this just
arranges for the generic code to save those instead of needing the
arm backend to do it manually.

2020-09-14  Jakub Jelinek  <jakub@redhat.com>

	* config/arm/arm.opt (x_arm_arch_string, x_arm_cpu_string,
	x_arm_tune_string): Remove TargetSave entries.
	(march=, mcpu=, mtune=): Add Save keyword.
	* config/arm/arm.c (arm_option_save): Remove.
	(TARGET_OPTION_SAVE): Don't redefine.
	(arm_option_restore): Don't restore x_arm_*_string here.