libstdc++: Check __cpp_exceptions in basic_string::reserve()

If exceptions are disabled then reallocating could abort, so ignore
shrink-to-fit requests.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.tcc [_GLIBCXX_USE_CXX11_ABI=0]
	(basic_string::reserve()): Do nothing if exceptions are not
	enabled.