arm64: elf: Enable BTI at exec based on ELF program properties

For BTI protection to be as comprehensive as possible, it is
desirable to have BTI enabled from process startup.  If this is not
done, the process must use mprotect() to enable BTI for each of its
executable mappings, but this is painful to do in the libc startup
code.  It's simpler and more sound to have the kernel do it
instead.

To this end, detect BTI support in the executable (or ELF
interpreter, as appropriate), via the
NT_GNU_PROGRAM_PROPERTY_TYPE_0 note, and tweak the initial prot
flags for the process' executable pages to include PROT_BTI as
appropriate.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>