target/arm: Handle FPU being disabled in FPCXT_NS accesses

target/arm: Handle FPU being disabled in FPCXT_NS accesses

If the guest makes an FPCXT_NS access when the FPU is disabled,
one of two things happens:
 * if there is no active FP context, then the insn behaves the
   same way as if the FPU was enabled: writes ignored, reads
   same value as FPDSCR_NS
 * if there is an active FP context, then we take a NOCP
   exception

Add code to the sysreg read/write functions which emits
code to take the NOCP exception in the latter case.

At the moment this will never be used, because the NOCP checks in
m-nocp.decode happen first, and so the trans functions are never
called when the FPU is disabled.  The code will be needed when we
move the sysreg access insns to before the NOCP patterns in the
following commit.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210618141019.10671-3-peter.maydell@linaro.org