asm-generic: Add a generic set_handle_irq

I've copied this from arm64, but it appears to be the same code that's
in arm and openrisc.  I wanted to use it for RISC-V's interrupt handler
as well, so despite this only being a handful of lines of code it seems
worth having a generic version -- if it existed when we created our
interrupt controller then maybe we would have just started with the
generic version.

This patch alone is just dead code, further patches actually use this
infrastructure.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>