extras/tools: Patch libtool to find the compiler-rt that clang uses by default

When libtool links DLLs that include C++ code, it does so by
passing -nostdlib and manually adding the compiler built-in
default libraries. When linking, clang specifies the compiler-rt
library by passing the full path to the .a file, instead of using
-L + -l. libtool currently doesn't pick up such a full path to
a static library as a library it should include.

This patch is a custom libtool patch from chromium os [1],
upstreaming is in progress at [2].

[1] https://chromium.googlesource.com/chromiumos/overlays/portage-stable/+/88201a517e3a797e4197bacc4959785a934c08d6
[2] https://debbugs.gnu.org/cgi/bugreport.cgi?bug=27866

extras/tools/libtool-2.4.6-clang-libs.patch

+Libtool checks only for libraries linked as -l* when trying to
+find internal compiler libraries. Clang, however uses the absolute
+path to link its internal libraries e.g. compiler_rt. This patch
+handles clang's statically linked libraries when finding internal
+compiler libraries.
+https://crbug.com/749263
+https://debbugs.gnu.org/cgi/bugreport.cgi?bug=27866
+
+--- a/libltdl/m4/libtool.m4
++++ b/libltdl/m4/libtool.m4
+@@ -7531,7 +7544,7 @@
+   for p in `eval "$output_verbose_link_cmd"`; do
+     case $prev$p in
+
+-    -L* | -R* | -l*)
++    -L* | -R* | -l* | */libclang_rt.*.a)
+        # Some compilers place space between "-{L,R}" and the path.
+        # Remove the space.
+        if test x-L = "$p" ||
+

extras/tools/tools.mak

@@ -88,6 +88,7 @@ libtool: libtool-$(LIBTOOL_VERSION).tar.gz
 	$(UNPACK)
 	$(APPLY) libtool-2.4.2-bitcode.patch
 	$(APPLY) libtool-2.4.2-san.patch
+	$(APPLY) libtool-2.4.6-clang-libs.patch
 	$(MOVE)
 
 .libtool: libtool .automake