Add stdarch to the list of ignored prefixes

Since libgrust/rustc-lib/stdarch is a library copied from rustc, we
should include the directory in ignored_prefixes.

contrib/ChangeLog:

	* gcc-changelog/git_commit.py (ignored_prefixes): Add
	'libgrust/rustc-lib/stdarch/'.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>

contrib/gcc-changelog/git_commit.py

@@ -151,6 +151,7 @@ ignored_prefixes = {
     'libffi/',
     'libgo/',
     'libgrust/rustc-lib/core/',
+    'libgrust/rustc-lib/stdarch/',
     'libphobos/libdruntime/',
     'libphobos/src/',
     'libsanitizer/',