kselftest/arm64: Enable GCS for the FP stress tests

kselftest/arm64: Enable GCS for the FP stress tests

While it's a bit off topic for them the floating point stress tests do give
us some coverage of context thrashing cases, and also of active signal
delivery separate to the relatively complicated framework in the actual
signals tests. Have the tests enable GCS on startup, ignoring failures so
they continue to work as before on systems without GCS.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Tested-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-39-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>