Set binaryen's git branch to `main` in manifest. (#684)

Binaryen renamed their default branch to main from master.
This breaks builds of the SDK from the sources.

This commit retargets binaryen builds to use the git branch `main` when appropriate.
The version name "master" remains the same to maintain compatibility with existing build scripts

Fixes #683.
diff --git a/emsdk_manifest.json b/emsdk_manifest.json
index 0bb0e48..4a613b9 100644
--- a/emsdk_manifest.json
+++ b/emsdk_manifest.json
@@ -420,7 +420,7 @@
     "bitness": 32,
     "append_bitness": false,
     "url": "https://github.com/WebAssembly/binaryen.git",
-    "git_branch": "master",
+    "git_branch": "main",
     "activated_cfg": "BINARYEN_ROOT='%installation_dir%%generator_prefix%_32bit_binaryen'",
     "activated_path": "%installation_dir%%generator_prefix%_32bit_binaryen/bin",
     "activated_env": "BINARYEN_ROOT=%installation_dir%%generator_prefix%_32bit_binaryen",
@@ -435,7 +435,7 @@
     "bitness": 64,
     "append_bitness": false,
     "url": "https://github.com/WebAssembly/binaryen.git",
-    "git_branch": "master",
+    "git_branch": "main",
     "activated_cfg": "BINARYEN_ROOT='%installation_dir%%generator_prefix%_64bit_binaryen'",
     "activated_path": "%installation_dir%%generator_prefix%_64bit_binaryen/bin",
     "activated_env": "BINARYEN_ROOT=%installation_dir%%generator_prefix%_64bit_binaryen",