Fix .gitignore ignores part of the committed repo.

(cherry picked from commit 37d68929e8ab3a1cab550a4cb83152eb13012f8c)

.gitignore

@@ -246,9 +246,6 @@ xcuserdata/
 x64/
 x86/
 
-# Do not ignore x86 folders anywhere under thirdparty libraries
-!thirdparty/**/x86/
-
 [Ww][Ii][Nn]32/
 [Aa][Rr][Mm]/
 [Aa][Rr][Mm]64/
@@ -258,6 +255,12 @@ bld/
 [Ll]og/
 [Ll]ogs/
 
+# Do not ignore arch-specific folders anywhere under thirdparty libraries
+!thirdparty/**/x64/
+!thirdparty/**/x86/
+!thirdparty/**/arm/
+!thirdparty/**/arm64/
+
 # Visual Studio 2015/2017 cache/options directory
 .vs/