x86_64: fix RBX clobber in nested IRQ case

In the code path for nested interrupts, we are not saving
RBX, yet the assembly code is using it as a storage location
for the ISR.

Use RAX. It is backed up in both the nested and non-nested
cases, and the ASM code is not currently using it at that
point.

Fixes: #29594

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>