libstdc++: Do not include <cxxabi.h> in <stacktrace>

This avoids polluting the global namespace with the "abi" namespace
alias.

libstdc++-v3/ChangeLog:

	* include/std/stacktrace: Do not include <cxxabi.h>.
	(__cxa_demangle): Declare.

libstdc++-v3/include/std/stacktrace

@@ -38,7 +38,6 @@
 #include <bits/stl_iterator.h>
 #include <bits/stl_uninitialized.h>
 #include <ext/numeric_traits.h>
-#include <cxxabi.h>
 
 struct __glibcxx_backtrace_state;
 struct __glibcxx_backtrace_simple_data;
@@ -70,6 +69,13 @@ __glibcxx_backtrace_syminfo(__glibcxx_backtrace_state*, uintptr_t addr,
 			    void*);
 }
 
+namespace __cxxabiv1
+{
+  extern "C" char*
+  __cxa_demangle(const char* __mangled_name, char* __output_buffer,
+		 size_t* __length, int* __status);
+}
+
 namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION