libstdc++: Make std::error_code printer more robust

This attempts to implement a partial workaround for the GDB bug
https://sourceware.org/bugzilla/show_bug.cgi?id=28856 which causes GDB
to crash when printing a frame with a std::error_code argument.

By recognising the known error categories defined in the library and
hardcoding their names we do not need to call cat->name() on the
category.  This has the additional benefit of also working when
debugging a core file rather than a running process. For those known
categories we can also cast the int value to the corresponding error
code enum (e.g. future_errc) so that we show an enumerator instead of
just an integer.

For program-defined categories we just use the name of the dynamic type
to identify the category, and print the value as an integer. Once the
GDB bug is fixed and the virtual name() function can be called safely,
that would be preferable. For now it's better to have an imperfect
printer that doesn't crash GDB.

This rewritten StdErrorCodePrinter needs gdb.Value.dynamic_type, so is
only registered if that is supported, which means GDB 7.7 and later.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (StdErrorCodePrinter): Replace
	code that call cat->name() on std::error_category objects.
	Identify known categories by symbol name and use a hardcoded
	name. Print error code values as enumerators where appopriate.
	* testsuite/libstdc++-prettyprinters/cxx11.cc: Adjust expected
	name of custom category. Check io_errc and future_errc errors.

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