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