x86-32: improve atomic64_t functions (v2)

Changes in v2:
- 386/486 is supported with a custom assembly implementation, the generic
  implementation is no longer used/modified
- dropped SSE code
- changed CALL alternative code to use a custom alternative type:
  insn parser no longer used
- several implementation improvements
- several formatting/style improvements
- merged 386 support into main patch

This patchset improves the atomic64_t functions on x86-32.
It also includes a testsuite that has been used to test this functionality
and can test any atomic64_t implementation.

It offers the following improvements:
1. Better code due to hand-written assembly (e.g. use of the ZF flag)
2. All atomic64 functions implemented
3. Support for 386/486 due to the ability to alternatively use either
   the cmpxchg8b assembly implementation or the 386 cli/popf assembly one

The first patches add functionality to the alternatives system to support
the new atomic64_t code.
A patch that improves cmpxchg64() using that functionality is also included.

To test this code, enable CONFIG_ATOMIC64_SELFTEST, compile for 386 and
boot normally and with "clearcpuid=8".

You should receive a message stating that the atomic64 test passed,
along with the selected configuration.

386/486 SMP is not supported, following existing practice, but the code
is structured to allow to very easily add such support.

Signed-off-by: Luca Barbieri <luca@luca-barbieri.com>