The go tool will pass -I objdir as one of the flags, where objdir is
the temporary build directory. Remove that from _cgo_flags: we don't
need it, and it will be different each time.
Sort the flags to avoid the unpredictable map iteration order.
This matters for gccgo because for a package that uses cgo, the go
tool when building for gccgo will store the _cgo_flags file in the
archive. That means that we want to generate identical _cgo_flags for
every run.
The test for this is the cmd/go testsuite, to follow in a future CL.
Reviewed-on: https://go-review.googlesource.com/45692
From-SVN: r249199
ea9a08f5df