target/arm: Add and use defines for EXCRET constants

The exception-return magic values get some new bits in v8M, which
makes some bit definitions for them worthwhile.

We don't use the bit definitions for the switch on the low bits
which checks the return type for v7M, because this is defined
in the v7M ARM ARM as a set of valid values rather than via
per-bit checks.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1505137930-13255-7-git-send-email-peter.maydell@linaro.org
