change bazel cpu symbol from wasm to wasm32

this is referred from https://github.com/bazelbuild/platforms/blob/master/cpu/BUILD#L78

Change-Id: I2e33f45ee05c5e8d63104de98de15fb21ddc23e6
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57470
Reviewed-by: Paul Wankadia <junyer@google.com>

BUILD

diff --git a/BUILD b/BUILD
index 8a1a9c0..3dc27d5 100644
--- a/BUILD
+++ b/BUILD
@@ -15,7 +15,7 @@
 
 config_setting(
     name = "wasm",
-    values = {"cpu": "wasm"},
+    values = {"cpu": "wasm32"},
 )
 
 config_setting(