Allocate GCS based on the stack size, this can be used for coroutines
(makecontext) and thread creation (if the kernel allows user allocated
GCS).

Unlike for BTI, the kernel does not process GCS properties so update
GL(dl_aarch64_gcs) before the GCS status is set.

check_gcs is called for each dependency of a DSO, but the GNU property
of the ld.so is not processed so ldso->l_mach.gcs may not be correct.
Just assume ld.so is GCS compatible independently of the ELF marking.

Allows using the same function for static exe.

Allows using the same function for static exe.

policy sets how gcs tunable and gcs marking turns into gcs state:

0: state = tunable
1: state = marking ? tunable : (tunable && dlopen ? err : 0)
2: state = marking ? tunable : (tunable ? err : 0)

Use the dynamic linker start code to enable GCS in the dynamic linked
case after _dl_start returns and before _dl_start_user which marks
the point after which user code may run.

Like in the static linked case this ensures that GCS is enabled on a
top level stack frame.