TODO: this is just for the arm/gcs branch while it is being developed

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.

TODO: not clear if the two are always equivalent other than the
ordering and in case of static linking when dl-support.c leaves
l_initfini NULL.

Allows using the same function for static exe.

TODO: not clear if the two are always equivalent other than the
ordering and in case of static linking when dl-support.c leaves
l_initfini NULL.

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)

TODO: state lock, default policy

TODO: binutils config check
TODO: build attributes instead of gnu property

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.

Use the ARCH_SETUP_TLS hook to enable GCS in the static linked case.
The system call must be inlined and then GCS is enabled on a top
level stack frame that does not return and has no exception handlers
above it.