hw/arm/boot: Set SCR_EL3.HXEn when booting kernel

hw/arm/boot: Set SCR_EL3.HXEn when booting kernel

When we direct boot a kernel on a CPU which emulates EL3, we need to
set up the EL3 system registers as the Linux kernel documentation
specifies:
     https://www.kernel.org/doc/Documentation/arm64/booting.rst

For CPUs with FEAT_HCX support this includes:
    - SCR_EL3.HXEn (bit 38) must be initialised to 0b1.

but we forgot to do this when implementing FEAT_HCX, which would mean
that a guest trying to access the HCRX_EL2 register would crash.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20221027140207.413084-3-peter.maydell@linaro.org