ada: Port System.Stack_Usage to CHERI

ada: Port System.Stack_Usage to CHERI

This unit performed integer to address conversions to calculate stack addresses
which, on a CHERI target, result in an invalid capability that triggers a
capability tag fault when dereferenced during stack filling. This patch updates
the unit to preserve addresses (capabilities) during the calculations.

The method used to determine the stack base address is also updated to CHERI.
The current method tries to get the stack base from the compiler info for the
current task. If no info is found, then as a fallback it estimates the base by
taking the address of a variable on the stack. This address is then derived to
calculate the range of addresses to fill the stack.

This fallback does not work on CHERI since taking the 'Address of a stack variable
will result in a capability with bounds restricted to that object and attempting to
write outside those bounds triggers a capability bounds fault. Instead, we add a
new function Get_Stack_Base which, on CHERI, gets the exact stack base from the
upper bound of the capability stack pointer (CSP) register. On non-CHERI platforms,
Get_Stack_Base returns the stack base from the compiler info, resulting in the same
behaviour as before on those platforms.

gcc/ada/ChangeLog:

	* Makefile.rtl (LIBGNAT_TARGET_PAIRS): New unit s-tsgsba__cheri.adb for morello-freebsd.
	* libgnarl/s-tassta.adb (Get_Stack_Base): New function.
	* libgnarl/s-tsgsba__cheri.adb: New file for CHERI targets.
	* libgnarl/s-tsgsba.adb: New default file for non-CHERI targets.
	* libgnat/s-stausa.adb (Fill_Stack, Compute_Result): Port to CHERI.
	* libgnat/s-stausa.ads (Initialize_Analyzer, Stack_Analyzer): Port to CHERI.