libstdc++: Define __glibcxx_assert_fail for non-verbose build [PR115585]

libstdc++: Define __glibcxx_assert_fail for non-verbose build [PR115585]

When the library is configured with --disable-libstdcxx-verbose the
assertions just abort instead of calling __glibcxx_assert_fail, and so I
didn't export that function for the non-verbose build. However, that
option is documented to not change the library ABI, so we still need to
export the symbol from the library. It could be needed by programs
compiled against the headers from a verbose build.

The non-verbose definition can just call abort so that it doesn't pull
in I/O symbols, which are unwanted in a non-verbose build.

libstdc++-v3/ChangeLog:

	PR libstdc++/115585
	* src/c++11/assert_fail.cc (__glibcxx_assert_fail): Add
	definition for non-verbose builds.