libstdc++: Restore support for including <name.h> in extern "C" [PR115797]

The r15-1857 change means that <type_traits> is included by <cmath> for
C++17 and up, which breaks code including <math.h> inside an extern "C"
block. Although doing that is not allowed by the C++ standard, there's
lots of existing code which incorrectly thinks it's a good idea and so
we try to support it.

libstdc++-v3/ChangeLog:

	PR libstdc++/115797
	* include/std/type_traits: Ensure "C++" language linkage.
	* testsuite/17_intro/headers/c++2011/linkage.cc: Replace
	dg-options with c++11 target selector.