target/arm: Add GCS cpregs

target/arm: Add GCS cpregs

Add isar_feature_aa64_gcs.
Enable SCR_GCSEN in scr_write.
Enable HCRX_GCSEN in hcrx_write.
Default HCRX_GCSEN on if EL2 disabled.
Add the GCSCR* and GCSPR* registers.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20251008215613.300150-32-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>