selftests: arm64: Factor out utility functions for assembly FP tests

The various floating point test programs written in assembly have a bunch
of helper functions and macros which are cut'n'pasted between them. Factor
them out into a separate source file which is linked into all of them.

We don't include memcmp() since it isn't as generic as it should be and
directly branches to report an error in the programs.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211019181851.3341232-1-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>