math: Handle fabsf128 !__USE_EXTERN_INLINES

Work around the clang limitation wrt inline function and attribute
definition, where it does not allow to 'add' new attribute if a
function is already defined:

clang on x86_64 fails to build s_fabsf128.c with:

../sysdeps/ieee754/float128/../ldbl-128/s_fabsl.c:32:1: error: attribute declaration must precede definition [-Werror,-Wignored-attributes]
   32 | libm_alias_ldouble (__fabs, fabs)
      | ^
../sysdeps/generic/libm-alias-ldouble.h:63:38: note: expanded from macro 'libm_alias_ldouble'
   63 | #define libm_alias_ldouble(from, to) libm_alias_ldouble_r (from, to, )
      |                                      ^
../sysdeps/ieee754/float128/float128_private.h:133:43: note: expanded from macro 'libm_alias_ldouble_r'
  133 | #define libm_alias_ldouble_r(from, to, r) libm_alias_float128_r (from, to, r)
      |                                           ^
../sysdeps/ieee754/float128/s_fabsf128.c:5:3: note: expanded from macro 'libm_alias_float128_r'
    5 |   static_weak_alias (from ## f128 ## r, to ## f128 ## r);       \
      |   ^
./../include/libc-symbols.h:166:46: note: expanded from macro 'static_weak_alias'
  166 | #  define static_weak_alias(name, aliasname) weak_alias (name, aliasname)
      |                                              ^
./../include/libc-symbols.h:154:38: note: expanded from macro 'weak_alias'
  154 | # define weak_alias(name, aliasname) _weak_alias (name, aliasname)
      |                                      ^
./../include/libc-symbols.h:156:52: note: expanded from macro '_weak_alias'
  156 |   extern __typeof (name) aliasname __attribute__ ((weak, alias (#name))) \
      |                                                    ^
../include/math.h:134:1: note: previous definition is here
  134 | fabsf128 (_Float128 x)

If compiler does not support __USE_EXTERN_INLINES we need to route
fabsf128 call to an internal symbol.

include/math.h

@@ -114,9 +114,11 @@ __issignalingf (float x)
 
 # if __HAVE_DISTINCT_FLOAT128
 
+#  ifdef __USE_EXTERN_INLINES
+
 /* __builtin_isinf_sign is broken in GCC < 7 for float128.  */
-#  if ! __GNUC_PREREQ (7, 0)
-#   include <ieee754_float128.h>
+#   if ! __GNUC_PREREQ (7, 0)
+#    include <ieee754_float128.h>
 extern inline int
 __isinff128 (_Float128 x)
 {
@@ -126,13 +128,16 @@ __isinff128 (_Float128 x)
   lx |= -lx;
   return ~(lx >> 63) & (hx >> 62);
 }
-#  endif
+#   endif
 
 extern inline _Float128
 fabsf128 (_Float128 x)
 {
   return __builtin_fabsf128 (x);
 }
+#  else
+libm_hidden_proto (fabsf128)
+#  endif
 # endif
 
 

sysdeps/ieee754/float128/s_fabsf128.c

@@ -1,2 +1,5 @@
 #include <float128_private.h>
 #include "../ldbl-128/s_fabsl.c"
+#ifndef __USE_EXTERN_INLINES
+libm_hidden_def (fabsf128)
+#endif