Currently the directory is created only if -objdir is omited.
Creating the directory here is useful to avoid doing this in each build system.
And also this is consistent with similar flags of other tools like `-o`.
Change-Id: Ic39d6eb3e003bc4884089f80f790e30df4a54b01
Reviewed-on: https://go-review.googlesource.com/c/go/+/576815 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>