Inform the Fuchsia GN SDK about the Fuchsia toolchain's version of readelf.

Avoids the use of the system's readelf, which emits warning about newer DWARF features it doesn't know about.

Change-Id: I6f6be5670bba591bfa1a5fa0bf9c53da28020b08
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/508165
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

.gn

@@ -15,3 +15,8 @@ secondary_source = "//build/secondary/"
 
 # Override the default script executable to always be python3.
 script_executable = "python3"
+
+default_args = {
+  # Overwrite default args declared in the Fuchsia sdk
+  fuchsia_sdk_readelf_exec = "//buildtools/linux-x64/clang/bin/llvm-readelf"
+}