Powerpc on core-updates

[Andreas Enge <andreas@enge.fr>]

Hello,

recently I claimed that powerpc was repaired, but I must have made a mistake.
It is still completely broken:
   https://ci.guix.gnu.org/eval/391720/dashboard?system=powerpc64le-linux
due to this:
   https://issues.guix.gnu.org/61879
 
It does not look easy to fix, but might be *the* blocker for a core-updates
merge...

The error is this:
../../../libstdc++-v3/src/c++17/floating_from_chars.cc: In function 'std::from_chars_result std::from_chars(const char*, const char*, __ieee128&, std::chars_format)':
../../../libstdc++-v3/src/c++17/floating_from_chars.cc:499:8: error: 'string' is not a member of 'std::pmr'; did you mean 'std::string'?
  499 |   pmr::string buf(&mr);
      |        ^~~~~~
In file included from /tmp/guix-build-libstdc++-11.3.0.drv-0/gcc-11.3.0/build/include/string:39,
                 from ../../../libstdc++-v3/src/c++17/floating_from_chars.cc:34:
/tmp/guix-build-libstdc++-11.3.0.drv-0/gcc-11.3.0/build/include/bits/stringfwd.h:79:33: note: 'std::string' declared here
   79 |   typedef basic_string<char>    string;
      |                                 ^~~~~~
../../../libstdc++-v3/src/c++17/floating_from_chars.cc:504:55: error: 'buf' was not declared in this scope
  504 |       if (const char* pat = pattern(first, last, fmt, buf)) [[likely]]

In the file
   libstdc++-v3/src/c++17/floating_from_chars.cc
previous functions have code like this:
#if _GLIBCXX_USE_CXX11_ABI
  buffer_resource mr;
  pmr::string buf(&mr);
#else
  string buf;
  if (!reserve_string(buf))
    return make_result(first, 0, {}, ec);
#endif

while here we only have:
  buffer_resource mr;
  pmr::string buf(&mr);

So my guess would be that we should simply replace this snippet with the
one above.

Could someone with access to a powerpc machine try out this change?

Andreas