opal/asm: fall back on inline asm atomics in some cases

This commit changes the asm configure logic to fall back on inline asm
atomics on systems that 1) have __sync atomics, 2) do not have 64-bit
__sync atomics, and 3) support 64-bit asm.

Signed-off-by: Nathan Hjelm <hjelmn@me.com>

config/opal_config_asm.m4

@@ -110,6 +110,9 @@ __sync_add_and_fetch(&tmp, 1);],
 
   AC_DEFINE_UNQUOTED([OPAL_ASM_SYNC_HAVE_64BIT],[$opal_asm_sync_have_64bit],
 		     [Whether 64-bit is supported by the __sync builtin atomics])
+
+  # Check for 128-bit support
+  OPAL_CHECK_SYNC_BUILTIN_CSWAP_INT128
 ])
 
 
@@ -893,7 +896,6 @@ AC_DEFUN([OPAL_CONFIG_ASM],[
     opal_cv_asm_builtin="BUILTIN_NO"
     if test "$opal_cv_asm_builtin" = "BUILTIN_NO" && test "$enable_builtin_atomics" = "yes" ; then
        OPAL_CHECK_SYNC_BUILTINS([opal_cv_asm_builtin="BUILTIN_SYNC"], [])
-       OPAL_CHECK_SYNC_BUILTIN_CSWAP_INT128
     fi
     if test "$opal_cv_asm_builtin" = "BUILTIN_NO" && test "$enable_osx_builtin_atomics" = "yes" ; then
        AC_CHECK_HEADER([libkern/OSAtomic.h],
@@ -1024,6 +1026,12 @@ AC_MSG_ERROR([Can not continue.])
             ;;
         esac
 
+	if test "x$OPAL_ASM_SUPPORT_64BIT" = "x1" && test "$opal_cv_asm_builtin" = "BUILTIN_SYNC" &&
+		test "$opal_asm_sync_have_64bit" = "0" ; then
+	    # __sync builtins exist but do not implement 64-bit support. Fall back on inline asm.
+	    opal_cv_asm_builtin="BUILTIN_NO"
+	fi
+
       if test "$opal_cv_asm_builtin" = "BUILTIN_SYNC" ; then
         AC_DEFINE([OPAL_C_GCC_INLINE_ASSEMBLY], [1],
           [Whether C compiler supports GCC style inline assembly])