arm: realm: Add test for FPU/SIMD context save/restore

Test that the FPU/SIMD registers are saved and restored correctly when
context switching VCPUs.

In order to test fpu/simd functionality, we need to make sure that
kvm-unit-tests doesn't generate code that uses the fpu registers, as that
might interfere with the test results. Thus make sure we compile the tests
with -mgeneral-regs-only.

Signed-off-by: Subhasish Ghosh <subhasish.ghosh@arm.com>
Signed-off-by: Joey Gouly <joey.gouly@arm.com>