cmd/go: allow -I= and -I$SYSROOT in cgo CFLAGS

Current checkFlags() didn't allow any not safe charactars in arguments.
In GCC "=" in arguments will be replaced with sysroot prefix, and used
by users to work with different SDK versions.

This CL allow to use "=" and $SYSROOT with -I argument.

Fixes #34449

Change-Id: I3d8b2b9d13251e454ea18e9d34a94b87c373c7b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/196783


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>