cmd/internal/obj/x86: avoid os.Chdir in issue19518_test.go

Chdir leaves the test in the wrong working directory if objdumpOutput
calls t.Fatalf (or panics), and it isn't necessary here anyway. Set
the Dir field on the commands instead.

Change-Id: I9f0eb0d4f8d15043f1e13472126ca1a1ce4b7cb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/167081


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>