configure.ac: rework -latomic check

configure.ac: rework -latomic check

The configure.ac logic added in commit
2ef7f23820a67e958c2252bd81eb0458903ebf33 ("configure: check if
-latomic is needed for __atomic_*") makes the assumption that if a
64-bit atomic intrinsic test program fails to link without -latomic,
it is because we must use -latomic.

Unfortunately, this is not completely correct: libatomic only appeared
in gcc 4.8, and therefore gcc versions before that will not have
libatomic, and therefore don't provide atomic intrinsics for all
architectures. This issue was for example encountered on PowerPC with
a gcc 4.7 toolchain, where the build fails with:

powerpc-ctng_e500v2-linux-gnuspe/bin/ld: cannot find -latomic

This commit aims at fixing that, by not assuming -latomic is
available. The commit re-organizes the atomic intrinsics detection as
follows:

 (1) Test if a program using 64-bit atomic intrinsics links properly,
     without -latomic. If this is the case, we have atomic intrinsics,
     and we're good to go.

 (2) If (1) has failed, then test to link the same program, but this
     time with -latomic in LDFLAGS. If this is the case, then we have
     atomic intrinsics, provided we link with -latomic.

This has been tested in three situations:

 - On x86-64, where atomic instrinsics are all built-in, with no need
   for libatomic. In this case, config.log contains:

   GCC_ATOMIC_BUILTINS_SUPPORTED_FALSE='#'
   GCC_ATOMIC_BUILTINS_SUPPORTED_TRUE=''
   LIBATOMIC_LIBS=''

   This means: atomic intrinsics are available, and we don't need to
   link with libatomic.

 - On NIOS2, where atomic intrinsics are available, but some of them
   (64-bit ones) require using libatomic. In this case, config.log
   contains:

   GCC_ATOMIC_BUILTINS_SUPPORTED_FALSE='#'
   GCC_ATOMIC_BUILTINS_SUPPORTED_TRUE=''
   LIBATOMIC_LIBS='-latomic'

   This means: atomic intrinsics are available, and we need to link
   with libatomic.

 - On PowerPC with an old gcc 4.7 toolchain, where 32-bit atomic
   instrinsics are available, but not 64-bit atomic instrinsics, and
   there is no libatomic. In this case, config.log contains:

   GCC_ATOMIC_BUILTINS_SUPPORTED_FALSE=''
   GCC_ATOMIC_BUILTINS_SUPPORTED_TRUE='#'

   With means that atomic intrinsics are not usable.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>