arm: syscalls: fix some register issues

Upon return from a syscall handlers, the r1, r2, and r3 registers
could contain random kernel data that should not be leaked to user
mode. Zero these out before returning from _arm_do_syscall().
Fixes #7753.

The invocation macros need a clobber if r1, r2, or r3 are not used
to carry syscall arguments. This is a partial fix for #7754 but
there appear to be other issues.

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