fix cowpatch issue with libstdc++-v3 directory in gcc

this directory uses relative pathnames to access libgcc unwind
headers, but with cowpatch, those are relative to the libstdc++-v3
directory in the unpatched tree unless libstdc++-v3 has been
cow-split.

this calls for a general solution at some point, but for now, just add
an explicit dummy patch to trigger cow splitting.

patches/gcc-11.2.0/0006-cow-libstdc++v3.diff

@@ -0,0 +1,8 @@
+--- a/libstdc++-v3/ChangeLog	2024-03-13 18:04:21.050801063 -0400
++++ b/libstdc++-v3/ChangeLog	2024-03-13 18:04:25.665836804 -0400
+@@ -4068,4 +4068,4 @@
+ 
+ Copying and distribution of this file, with or without modification,
+ are permitted in any medium without royalty provided the copyright
+-notice and this notice are preserved.
++notice and this notice are preserved.