libstdc++: Avoid '-Wunused-parameter' for '__what' in function 'void std::__throw_for...

libstdc++: Avoid '-Wunused-parameter' for '__what' in function 'void std::__throw_format_error(const char*)'

In a '-fno-exceptions' configuration:

    In file included from ../../../../../source-gcc/libstdc++-v3/src/c++20/format.cc:29:
    [...]/build-gcc/[...]/libstdc++-v3/include/format: In function ‘void std::__throw_format_error(const char*)’:
    [...]/build-gcc/[...]/libstdc++-v3/include/format:200:36: error: unused parameter ‘__what’ [-Werror=unused-parameter]
      200 |   __throw_format_error(const char* __what)
          |                        ~~~~~~~~~~~~^~~~~~

	libstdc++-v3/
	* include/bits/c++config [!__cpp_exceptions]
	(_GLIBCXX_THROW_OR_ABORT): Reference '_EXC'.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>