arm64/sve: Add compile time checks for SVE hooks in generic functions

arm64/sve: Add compile time checks for SVE hooks in generic functions

The FPSIMD code was relying on IS_ENABLED() checks in system_suppors_sve()
to cause the compiler to delete references to SVE functions in some places,
add explicit IS_ENABLED() checks back.

Fixes: ef9c5d09797d ("arm64/sve: Remove redundant system_supports_sve() tests")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210415121742.36628-1-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>