target/arm: Honour M-profile FP enable bits

target/arm: Honour M-profile FP enable bits

Like AArch64, M-profile floating point has no FPEXC enable
bit to gate floating point; so always set the VFPEN TB flag.

M-profile also has CPACR and NSACR similar to A-profile;
they behave slightly differently:
 * the CPACR is banked between Secure and Non-Secure
 * if the NSACR forces a trap then this is taken to
   the Secure state, not the Non-Secure state

Honour the CPACR and NSACR settings. The NSACR handling
requires us to borrow the exception.target_el field
(usually meaningless for M profile) to distinguish the
NOCP UsageFault taken to Secure state from the more
usual fault taken to the current security state.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190416125744.27770-6-peter.maydell@linaro.org