Expand the INIT_SIGNALS and INIT_SIGHAND macros and remove

There doesn't seem to be any need to have the INIT_SIGNALS and INIT_SIGHAND
macros, so expand them in their single places of use and remove them.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Tony Luck <tony.luck@intel.com>
Tested-by: Will Deacon <will.deacon@arm.com> (arm64)
Tested-by: Palmer Dabbelt <palmer@sifive.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>