Add missing android extensions (#813)

Author: UebelAndre

rust/platform/triple_mappings.bzl

@@ -78,6 +78,7 @@ _SYSTEM_TO_BUILTIN_SYS_SUFFIX = {
 }
 
 _SYSTEM_TO_BINARY_EXT = {
+    "android": "",
     "darwin": "",
     "emscripten": ".js",
     "freebsd": "",
@@ -92,6 +93,7 @@ _SYSTEM_TO_BINARY_EXT = {
 }
 
 _SYSTEM_TO_STATICLIB_EXT = {
+    "android": ".a",
     "darwin": ".a",
     "emscripten": ".js",
     "freebsd": ".a",
@@ -103,6 +105,7 @@ _SYSTEM_TO_STATICLIB_EXT = {
 }
 
 _SYSTEM_TO_DYLIB_EXT = {
+    "android": ".so",
     "darwin": ".dylib",
     "emscripten": ".js",
     "freebsd": ".so",