Make common/ compile with Emscripten

Projects using Dawn that are compiled with Emscripten may use some
of our common headers and utilities. ex.) creating a ShaderModule
from SPIR-V. Add handling for __EMSCRIPTEN__ to Platform.h so
compiling to WASM works.

Also, implement GetExecutablePath on Emscripten as unreachable.
This is only ever used to search native paths in the Vulkan backend.

Bug: none
Change-Id: I5d9d6e27e287cb34e7af3214add1c4bc8c6a0f70
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/25064
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

src/common/Platform.h

@@ -41,6 +41,10 @@
 #    define DAWN_PLATFORM_FUCHSIA 1
 #    define DAWN_PLATFORM_POSIX 1
 
+#elif defined(__EMSCRIPTEN__)
+#    define DAWN_PLATFORM_EMSCRIPTEN 1
+#    define DAWN_PLATFORM_POSIX 1
+
 #else
 #    error "Unsupported platform."
 #endif
@@ -60,7 +64,7 @@
 #    define DAWN_PLATFORM_64_BIT 1
 static_assert(sizeof(sizeof(char)) == 8, "Expect sizeof(size_t) == 8");
 #elif defined(_WIN32) || defined(__arm__) || defined(__i386__) || defined(__mips32__) || \
-    defined(__s390__)
+    defined(__s390__) || defined(__EMSCRIPTEN__)
 #    define DAWN_PLATFORM_32_BIT 1
 static_assert(sizeof(sizeof(char)) == 4, "Expect sizeof(size_t) == 4");
 #else

src/common/SystemUtils.cpp

@@ -108,6 +108,11 @@ std::string GetExecutablePath() {
     // TODO: Implement on Fuchsia
     return "";
 }
+#elif defined(DAWN_PLATFORM_EMSCRIPTEN)
+std::string GetExecutablePath() {
+    UNREACHABLE();
+    return "";
+}
 #else
 #    error "Implement GetExecutablePath for your platform."
 #endif