Fix default float_t definition (bug 20855).

The default (top-level) version of bits/mathdef.h defines float_t to
double.  It is used on ColdFire, MicroBlaze, Nios II and SH3, all of
which define FLT_EVAL_METHOD to 0, so float_t should be float (and C11
requires a certain correspondence between these typedefs and
FLT_EVAL_METHOD values).

I proposed fixing this default in
<https://sourceware.org/ml/libc-alpha/2015-01/msg00499.html>, with no
objections from architecture maintainers, and this patch makes that
fix.  As noted in the NEWS entry added, this might affect the ABIs of
non-glibc libraries (ImageMagick has been mentioned in gcc-patches
discussion of the S/390 case - which is unaffected by this patch), but
as noted in my previous message, affected libraries would have
problems with -mfpmath=sse anyway on 32-bit x86.

A (compilation) testcase is added to verify the required
correspondence of typedefs to FLT_EVAL_METHOD values.  This test is
built with -fexcess-precision=standard to avoid any issues with GCC 7
on S/390 providing a more accurate FLT_EVAL_METHOD definition in the
default (no excess precision) mode.  (This will also be usable to test
a fix for the recently reported bug about these typedefs on x86_64
-mfpmath=387, as architecture-specific tests can be added that

It is entirely possible that the fixed default makes some
architecture-specific versions of bits/mathdef.h semantically
equivalent to the default version and so no longer required.  I don't
intend to investigate that separately from the refactoring I proposed
in <https://sourceware.org/ml/libc-alpha/2016-11/msg00745.html>, which
will create as few header variants as possible for each group of
definitions.

Tested (compilation only) with build-many-glibcs.py.

	[BZ #20855]
	* bits/mathdef.h (float_t): Define to float.
	* math/test-flt-eval-method.c: New file.
	* math/Makefile (tests): Add test-flt-eval-method.
	(CFLAGS-test-flt-eval-method.c): New variable.

1 files changed, 8 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index a475194..57935c6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2016-11-23  Joseph Myers  <joseph@codesourcery.com>
+
+	[BZ #20855]
+	* bits/mathdef.h (float_t): Define to float.
+	* math/test-flt-eval-method.c: New file.
+	* math/Makefile (tests): Add test-flt-eval-method.
+	(CFLAGS-test-flt-eval-method.c): New variable.
+
 2016-11-22  Steve Ellcey  <sellcey@caviumnetworks.com>
 
 	* sysdeps/unix/sysv/linux/fstatfs64.c: Hide prototypes for fstatfs