c: Adjust LDBL_EPSILON for C2x for IBM long double

C2x changes the <float.h> definition of *_EPSILON to apply only to
normalized numbers.  The effect is that LDBL_EPSILON for IBM long
double becomes 0x1p-105L instead of 0x1p-1074L.

There is a reasonable case for considering this a defect fix - it
originated from the issue reporting process (DR#467), though it ended
up being resolved by a paper (N2326) for C2x rather than through the
issue process, and code using *_EPSILON often needs to override the
pre-C2x value of LDBL_EPSILON and use something on the order of
magnitude of the C2x value instead.  However, I've followed the
conservative approach of only making the change for C2x and not for
previous standard versions (and not for C++, which doesn't have the
C2x changes in this area).

The testcases added are intended to be valid for all long double
formats.  The C11 one is based on
gcc.target/powerpc/rs6000-ldouble-2.c (and when we move to a C2x
default, gcc.target/powerpc/rs6000-ldouble-2.c will need an
appropriate option added to keep using an older language version).

Tested with no regressions for cross to powerpc-linux-gnu.

gcc/c-family/
	* c-cppbuiltin.cc (builtin_define_float_constants): Do not
	special-case __*_EPSILON__ setting for IBM long double for C2x.

gcc/testsuite/
	* gcc.dg/c11-float-7.c, gcc.dg/c2x-float-12.c: New tests.

gcc/c-family/c-cppbuiltin.cc

@@ -279,7 +279,7 @@ builtin_define_float_constants (const char *name_prefix,
   /* The difference between 1 and the least value greater than 1 that is
      representable in the given floating point type, b**(1-p).  */
   sprintf (name, "__%s_EPSILON__", name_prefix);
-  if (fmt->pnan < fmt->p)
+  if (fmt->pnan < fmt->p && (c_dialect_cxx () || !flag_isoc2x))
     /* This is an IBM extended double format, so 1.0 + any double is
        representable precisely.  */
       sprintf (buf, "0x1p%d", fmt->emin - fmt->p);

gcc/testsuite/gcc.dg/c11-float-7.c

@@ -0,0 +1,24 @@
+/* Test C11 definition of LDBL_EPSILON.  Based on
+   gcc.target/powerpc/rs6000-ldouble-2.c.  */
+/* { dg-do run } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+#include <float.h>
+
+extern void abort (void);
+extern void exit (int);
+
+int
+main (void)
+{
+  volatile long double ee = 1.0;
+  long double eps = ee;
+  while (ee + 1.0 != 1.0)
+    {
+      eps = ee;
+      ee = eps / 2;
+    }
+  if (eps != LDBL_EPSILON)
+    abort ();
+  exit (0);
+}

gcc/testsuite/gcc.dg/c2x-float-12.c

@@ -0,0 +1,19 @@
+/* Test C2x definition of LDBL_EPSILON.  */
+/* { dg-do run } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+#include <float.h>
+
+extern void abort (void);
+extern void exit (int);
+
+int
+main (void)
+{
+  volatile long double x = 1.0L;
+  for (int i = 0; i < LDBL_MANT_DIG - 1; i++)
+    x /= 2;
+  if (x != LDBL_EPSILON)
+    abort ();
+  exit (0);
+}