PCI/MSI: Provide a new set of mask and unmask functions

The existing mask/unmask functions are convoluted and generate suboptimal
assembly code.

Provide a new set of functions which will be used in later patches to
replace the exisiting ones.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/875ywetozb.ffs@tglx