Fix Wasm toolchain compiler path on Windows

On Windows CMD, a path starting with "../" is not a valid executable:

>../../third_party/emsdk/upstream/emscripten/em++.bat --version
'..' is not recognized as an internal or external command,
operable program or batch file.

Change the rebase_path invocations so they will use "system-absolute
native style paths with system path separators", e.g.:

>C:/src/dawn/third_party/emsdk/upstream/emscripten/em++.bat

Fixes building Dawn's Wasm GN targets on Windows, tested locally.

Bug: 450675299
Change-Id: Ieae38d4cdb3d96f4f60599b55e7f593c9247d354
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7032591
Reviewed-by: Reid Kleckner <rnk@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Cr-Commit-Position: refs/heads/main@{#1528400}
NOKEYCHECK=True
GitOrigin-RevId: 13a3671b389e0355c210907aa58d95e8ced626a7

wasm/BUILD.gn

diff --git a/wasm/BUILD.gn b/wasm/BUILD.gn
index 4b4d65c..5bdcaca 100644
--- a/wasm/BUILD.gn
+++ b/wasm/BUILD.gn
@@ -19,9 +19,9 @@
     _suffix = ".bat"
   }
 
-  cc = rebase_path("$emscripten_path/emcc$_suffix", root_build_dir)
-  cxx = rebase_path("$emscripten_path/em++$_suffix", root_build_dir)
-  ar = rebase_path("$emscripten_path/emar$_suffix", root_build_dir)
+  cc = rebase_path("$emscripten_path/emcc$_suffix")
+  cxx = rebase_path("$emscripten_path/em++$_suffix")
+  ar = rebase_path("$emscripten_path/emar$_suffix")
   nm = cc
   ld = cxx