target-arm: Add WFx instruction trap support

Add support for trapping WFI and WFE instructions to the proper EL when
SCTLR/SCR/HCR settings apply.

Signed-off-by: Greg Bellows <greg.bellows@linaro.org>
[PMM: removed unnecessary tweaking of syn_wfx() prototype;
 use raise_exception();
 don't trap on WFE (and add comment explaining why not);
 remove unnecessary ARM_FEATURE checks;
 trap to EL3, not EL1, if in S-EL0 and SCTLR check fires]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
