tcg/ppc: Define _CALL_AIX for clang on ppc64(be)

Restructure the ifdef ladder, separating 64-bit from 32-bit,
and ensure _CALL_AIX is set for ELF v1.  Fixes the build for
ppc64 big-endian host with clang.

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tcg/ppc/tcg-target.c.inc

@@ -29,15 +29,24 @@
 /*
  * Standardize on the _CALL_FOO symbols used by GCC:
  * Apple XCode does not define _CALL_DARWIN.
- * Clang defines _CALL_ELF (64-bit) but not _CALL_SYSV (32-bit).
+ * Clang defines _CALL_ELF (64-bit) but not _CALL_SYSV or _CALL_AIX.
  */
-#if !defined(_CALL_SYSV) && \
-    !defined(_CALL_DARWIN) && \
-    !defined(_CALL_AIX) && \
-    !defined(_CALL_ELF)
-# if defined(__APPLE__)
+#if TCG_TARGET_REG_BITS == 64
+# ifdef _CALL_AIX
+    /* ok */
+# elif defined(_CALL_ELF) && _CALL_ELF == 1
+#  define _CALL_AIX
+# elif defined(_CALL_ELF) && _CALL_ELF == 2
+    /* ok */
+# else
+#  error "Unknown ABI"
+# endif
+#else
+# if defined(_CALL_SYSV) || defined(_CALL_DARWIN)
+    /* ok */
+# elif defined(__APPLE__)
 #  define _CALL_DARWIN
-# elif defined(__ELF__) && TCG_TARGET_REG_BITS == 32
+# elif defined(__ELF__)
 #  define _CALL_SYSV
 # else
 #  error "Unknown ABI"