target/arm: Allow reading flags from FPSCR for M-profile

rt==15 is a special case when reading the flags: it means the
destination is APSR. This patch avoids rejecting
vmrs apsr_nzcv, fpscr
as illegal instruction.

Cc: qemu-stable@nongnu.org
Signed-off-by: Christophe Lyon <christophe.lyon@linaro.org>
Message-id: 20191025095711.10853-1-christophe.lyon@linaro.org
[PMM: updated the comment]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>