opal_config_asm.m4: Fix the detection of 128 bits atomics.

Thanks to Stefan Teleman for identifying this issue and providing a
proof-of-concept patch.  We ended up revamping the detection of
128-bit atomics to reduce duplicated code and be a slightly simpler --
albiet perhaps a bit more verbose -- approach:

- Remove the --enable-cross-* options; they were confusing and
  unnecessary.
- Always try to compile / link the compiler-intrinsic 128-bit atomic
  functions.
  - Strengthen the C tests we use to be more robust.
  - Use m4 to avoid duplicating the C tests multiple times in the .m4
    source.
- If not cross-compiling, try to run a short test and ensure that they
  actually work (as of Aug 2018, there's at least one platform where
  they don't: clang 6 on ARM64).  If cross-compiling, just assume that
  they work.
- Add more comments about what is going on with all the tests; it's
  tricky stuff.  Our Future Selves will thank us.

Signed-off-by: George Bosilca <[email protected]>
Signed-off-by: Jeff Squyres <[email protected]>
(cherry picked from commit ff9df91)

Author: jsquyres