kselftest/arm64: Use the tools/include compiler.h rather than our own

The BTI test program started life as standalone programs outside the
kselftest suite so provided it's own compiler.h. Now that we have updated
the tools/include compiler.h to have all the definitions that we are using
and the arm64 selftsets pull in tools/includes let's drop our custom
version.

__unreachable() is named unreachable() there requiring an update in the
code.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230728-arm64-signal-memcpy-fix-v4-6-0c1290db5d46@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>