add support for __cxa_demangle

This adds support for __cxa_demangle through the cxxabi.h file.

At the moment it is the only symbol included.
The source was taken from FreeBSD
contrib/libcxxrt/libelftc_dem_gnu3.c
2176c9ab71c85efd90a6c7af4a9e04fe8e3d49ca

FreeBSD does say this is also almost verbatim from ELFToolkit

third_party/libcxx/cxxabi.h

@@ -0,0 +1,55 @@
+#ifndef _CXXABI_H
+#define _CXXABI_H 1
+
+#include "third_party/libcxx/cstddef"
+
+namespace abi {
+
+   /**
+   *  @brief Demangling routine.
+   *  ABI-mandated entry point in the C++ runtime library for demangling.
+   *
+   *  @param __mangled_name A NUL-terminated character string
+   *  containing the name to be demangled.
+   *
+   *  @param __output_buffer A region of memory, allocated with
+   *  malloc, of @a *__length bytes, into which the demangled name is
+   *  stored.  If @a __output_buffer is not long enough, it is
+   *  expanded using realloc.  @a __output_buffer may instead be null;
+   *  in that case, the demangled name is placed in a region of memory
+   *  allocated with malloc.
+   *
+   *  @param __length If @a __length is non-null, the length of the
+   *  buffer containing the demangled name is placed in @a *__length.
+   *
+   *  @param __status If @a __status is non-null, @a *__status is set to
+   *  one of the following values:
+   *   0: The demangling operation succeeded.
+   *  -1: A memory allocation failure occurred.
+   *  -2: @a mangled_name is not a valid name under the C++ ABI mangling rules.
+   *  -3: One of the arguments is invalid.
+   *
+   *  @return A pointer to the start of the NUL-terminated demangled
+   *  name, or a null pointer if the demangling fails.  The caller is
+   *  responsible for deallocating this memory using @c free.
+   *
+   *  The demangling is performed using the C++ ABI mangling rules,
+   *  with GNU extensions. For example, this function is used in
+   *  __gnu_cxx::__verbose_terminate_handler.
+   *
+   *  See https://gcc.gnu.org/onlinedocs/libstdc++/manual/ext_demangling.html
+   *  for other examples of use.
+   *
+   *  @note The same demangling functionality is available via
+   *  libiberty (@c <libiberty/demangle.h> and @c libiberty.a) in GCC
+   *  3.1 and later, but that requires explicit installation (@c
+   *  --enable-install-libiberty) and uses a different API, although
+   *  the ABI is unchanged.
+   */
+  extern "C" char*
+  __cxa_demangle(const char* __mangled_name, char* __output_buffer,
+		 size_t* __length, int* __status);
+
+}
+
+#endif

third_party/libcxx/libcxx.mk

@@ -67,6 +67,7 @@ THIRD_PARTY_LIBCXX_A_HDRS =					\
 	third_party/libcxx/ctype.h				\
 	third_party/libcxx/cwchar				\
 	third_party/libcxx/cwctype				\
+	third_party/libcxx/cxxabi.h				\
 	third_party/libcxx/deque				\
 	third_party/libcxx/errno.h				\
 	third_party/libcxx/exception				\
@@ -143,6 +144,7 @@ THIRD_PARTY_LIBCXX_A_SRCS_CC =					\
 	third_party/libcxx/chrono.cc				\
 	third_party/libcxx/condition_variable.cc		\
 	third_party/libcxx/condition_variable_destructor.cc	\
+	third_party/libcxx/demangle.cc	\
 	third_party/libcxx/exception.cc				\
 	third_party/libcxx/functional.cc			\
 	third_party/libcxx/future.cc				\
@@ -212,6 +214,16 @@ $(THIRD_PARTY_LIBCXX_A_OBJS): private				\
 			-ffunction-sections			\
 			-fdata-sections
 
+# Super gross! But I found no way to do -fno-error=permissive
+# this steps from the fact that demangle is a C++ file extension
+# but the source I copied was written in C.
+# The fix would be to go through all the casts and make them correct.
+o/$(MODE)/third_party/libcxx/demangle.o: private				\
+		CXXFLAGS +=							\
+			-fpermissive					\
+			-Wno-error
+
+
 THIRD_PARTY_LIBCXX_LIBS = $(foreach x,$(THIRD_PARTY_LIBCXX_ARTIFACTS),$($(x)))
 THIRD_PARTY_LIBCXX_SRCS = $(foreach x,$(THIRD_PARTY_LIBCXX_ARTIFACTS),$($(x)_SRCS))
 THIRD_PARTY_LIBCXX_HDRS = $(foreach x,$(THIRD_PARTY_LIBCXX_ARTIFACTS),$($(x)_HDRS))