arm64: update _current_cpu->nested properly

This is an uint32_t so the proper register width must be used, otherwise
the adjacent structure member will be overwritten (didn't happen in
practice because of struct member alignment but still). This makes the
inc_nest_counter and dec_nest_counter macros rather unwieldy, especially
with upcoming changes, so let's just remove them.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>