[gm.py] Add support for building mkgrokdump

Now you can do: "gm.py x64.release mkgrokdump"

NOTRY=true

Review-Url: https://codereview.chromium.org/2827383003
Cr-Commit-Position: refs/heads/master@{#44756}

tools/dev/gm.py

@@ -37,7 +37,7 @@ MODES = ["release", "debug", "optdebug"]
 # Modes that get built/run when you don't specify any.
 DEFAULT_MODES = ["release", "debug"]
 # Build targets that can be manually specified.
-TARGETS = ["d8", "cctest", "unittests", "v8_fuzzers"]
+TARGETS = ["d8", "cctest", "unittests", "v8_fuzzers", "mkgrokdump"]
 # Build targets that get built when you don't specify any (and specified tests
 # don't imply any other targets).
 DEFAULT_TARGETS = ["d8"]