Don't specify -pthread on Emscripten either.

Change-Id: I6b23bb04f0caaa2a65a23c941b1aea1eeb4da25d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62913
Reviewed-by: Ash Liu <almquist@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>

BUILD.bazel

@@ -58,6 +58,7 @@ cc_library(
         # WebAssembly support for threads is... fraught at every level.
         "@platforms//cpu:wasm32": [],
         "@platforms//cpu:wasm64": [],
+        "@platforms//os:emscripten": [],
         "@platforms//os:wasi": [],
         "@platforms//os:windows": [],
         "//conditions:default": ["-pthread"],
@@ -70,6 +71,7 @@ cc_library(
         # WebAssembly support for threads is... fraught at every level.
         "@platforms//cpu:wasm32": [],
         "@platforms//cpu:wasm64": [],
+        "@platforms//os:emscripten": [],
         "@platforms//os:wasi": [],
         "@platforms//os:windows": [],
         "//conditions:default": ["-pthread"],