GN: add a default build target

This target is built when no specific target is specified on the command
line. It avoids building all available targets. Specifically, the
optional use of LLVM as a JIT-compiler by SwiftShader was dominating
compile times.

See the GN reference for details:
https://gn.googlesource.com/gn/+/master/docs/reference.md#the-all-and-default-rules

For now, it's equivalent to the "all" target. Note the latter is an
explicit target, not the implicit one mentioned in the reference.

Bug: swiftshader:157
Change-Id: Ifd2750921a9cfc0163858da77a81c9eb33002fb2
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/39580
Auto-Submit: Nicolas Capens <nicolascapens@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

BUILD.gn

@@ -24,3 +24,9 @@ group("all") {
     deps += [ "examples:dawn_samples" ]
   }
 }
+
+# This target is built when no specific target is specified on the command line.
+group("default") {
+  testonly = true
+  deps = [ ":all" ]
+}