selftests/mm: Enable pkey_sighandler_tests on arm64

pkey_sighandler_tests.c makes raw syscalls using its own helper,
syscall_raw(). One of those syscalls is clone, which is problematic
as every architecture has a different opinion on the order of its
arguments.

To complete arm64 support, we therefore add an appropriate
implementation in syscall_raw(), and introduce a clone_raw() helper
that shuffles arguments as needed for each arch.

Having done this, we enable building pkey_sighandler_tests for arm64
in the Makefile.

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Link: https://lore.kernel.org/r/20241029144539.111155-6-kevin.brodsky@arm.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>