arch: arm: aarch32: Fix interrupt nesting

In the current interrupt nesting implementation, if an ISR is
interrupted while executing inside a branch, the lr_svc register will
be corrupted, and the branch of the interrupted ISR will exit to the
return address of the final branch of the interrupting ISR, which may
or may not correspond to the intended return address.

This commit fixes the aforementioned bug by storing the lr_svc register
in the stack at the ISR entry, and restoring its value before exiting
the ISR.

For more details, refer to the issue #30517.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>