Set default values for stack-clash and do basic validation in back-end.

Set default values for stack-clash and do basic validation in back-end.

This patch enforces that the default guard size for stack-clash protection for
AArch64 be 64KB unless the user has overriden it via configure in which case
the user value is used as long as that value is within the valid range.

It also does some basic validation to ensure that the guard size is only 4KB or
64KB and also enforces that for aarch64 the stack-clash probing interval is
equal to the guard size.

gcc/

	PR target/86486
	* config/aarch64/aarch64.c (aarch64_override_options_internal):
	Add validation for stack-clash parameters and set defaults.

From-SVN: r264753