arm_semihosting buildfix

arm_semihosting buildfix

The recent "add armv7m semihosting support" patch introduced two
build errors:

arm_semihosting.c: In function ‘do_semihosting’:
arm_semihosting.c:71: error: ‘spsr’ may be used uninitialized in this function
arm_semihosting.c:71: error: ‘lr’ may be used uninitialized in this function

This fixes those build errors.  The behavior is, however, untested.
(Also, note the two new REVISIT comments.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>