aarch64: Rework {inc,dec}_nest_counter

There are several issues with the current implemenation of the
{inc,dec}_nest_counter macros.

The first problem is that it's internally using a call to a misplaced
function called z_arm64_curr_cpu() (for some unknown reason hosted in
irq_manage.c) that could potentially clobber the caller-saved registers
without any notice to the user of the macro.

The second problem is that being a macro the clobbered registers should
be specified at the calling site, this is not possible given the current
implementation.

To fix these issues and make the call quicker, this patch rewrites the
code in assembly leveraging the availability of the _curr_cpu array. It
now clobbers only two registers passed from the calling site.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>