transaction_safe exceptions prevent libstdc++ building for some targets

[Joe Seymour]

../configure --target=msp430-elf --enable-languages=c,c++ && make -j4

Results in the msp430 -mlarge multilib failing to build with...

 > configure: error: Unknown underlying type for size_t
 > make[1]: *** [configure-target-libstdc++-v3] Error 1

This relates to...

 > commit 13143e139230dc4d72710a6c4c312105aeddce4f
 > Author: torvald <torvald@138bc75d-0d04-0410-961f-82ee72b054a4>
 > Date:  Fri Jan 15 22:42:41 2016 +0000
 >
 >    libstdc++: Make certain exceptions transaction_safe.

https://gcc.gnu.org/ml/libstdc++/2016-01/msg00015.html

...which iterates through a list of types at configure time, looking for one
matching __SIZE_TYPE__. The following patch (wip) allows the build to 
proceed
(to the next error), by adding __int20 to the list of types to try.

- Is this an acceptable solution?
- There's a comment above GLIBCXX_CHECK_SIZE_T_MANGLING, saying that it was
   copied from libitm. Should that be updated too, even if it's 
irrelevant for
   msp430?

Proceeding past that build error, all multilibs then fail to build with...

 > ../../../../../libstdc++-v3/src/c++11/cow-stdexcept.cc:274:3: error: 
static assertion failed: Pointers must be 32 bits or 64 bits wide
 >  static_assert(sizeof(uint64_t) == sizeof(void*)

The assert fails because msp430 has 16-bit or 20-bit pointers. The same 
error
can be reproduced for the rl78-elf target, which has 16-bit pointers.

Disabling the original changes for targets with unsupported pointer 
sizes seems
like a reasonable solution to me, but I can't see an existing mechanism 
to do
so? Do others agree?

More generally, it might be useful to have a mechanism to disable 
transactional
memory for some targets. I've observed things like register_tm_clones 
taking up
space in ARM Cortex-M binaries. Presumably, this would be achieved by 
adding an
--{enable,disable}-transactional-memory argument to configure? I'm happy to
work on a patch, if this is acceptable in principle.

Thanks,

2016-08-XX  Joe Seymour  <joe.s@somniumtech.com>

     * acinclude.m4 (GLIBCXX_CHCK_SIZE_T_MANGLING): Try __int20.
     * configure: Regenerate.

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index b0f88cb..7332d3e 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -4405,9 +4405,13 @@ AC_DEFUN([GLIBCXX_CHECK_SIZE_T_MANGLING], [
                         [extern __SIZE_TYPE__ x; extern unsigned long 
long x;],
                         [glibcxx_cv_size_t_mangling=y], [
            AC_TRY_COMPILE([],
-                         [extern __SIZE_TYPE__ x; extern unsigned short 
x;],
-                         [glibcxx_cv_size_t_mangling=t],
-                         [glibcxx_cv_size_t_mangling=x])
+                         [extern __SIZE_TYPE__ x; extern unsigned long 
long x;],
+                         [glibcxx_cv_size_t_mangling=y], [
+            AC_TRY_COMPILE([],
+                           [extern __SIZE_TYPE__ x; extern __int20 
unsigned x;],
+                           [glibcxx_cv_size_t_mangling=u6uint20],
+                           [glibcxx_cv_size_t_mangling=x])
+          ])
          ])
        ])
      ])
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index 41797a9..7ab84eb 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -80475,13 +80475,28 @@ else
  int
  main ()
  {
-extern __SIZE_TYPE__ x; extern unsigned short x;
+extern __SIZE_TYPE__ x; extern unsigned long long x;
    ;
    return 0;
  }
  _ACEOF
  if ac_fn_c_try_compile "$LINENO"; then :
-  glibcxx_cv_size_t_mangling=t
+  glibcxx_cv_size_t_mangling=y
+else
+
+            cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+extern __SIZE_TYPE__ x; extern __int20 unsigned x;
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  glibcxx_cv_size_t_mangling=u6uint20
  else
    glibcxx_cv_size_t_mangling=x
  fi
@@ -80497,6 +80512,9 @@ fi
  rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext

  fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+
+fi
  { $as_echo "$as_me:${as_lineno-$LINENO}: result: 
$glibcxx_cv_size_t_mangling" >&5
  $as_echo "$glibcxx_cv_size_t_mangling" >&6; }
    if test $glibcxx_cv_size_t_mangling = x; then