cmd/go: deflake TestScript/gcflags_patterns

The check below can fail incorrectly if the buildid ends with '-p'.

    ! stderr 'compile.* -e .*-p [^z]'

This fix changes regular expressions to '-e.* -p' or '-N.* -p' instead
of '-e .*-p'. '-l' is no longer used because the compiler accepts
multiple flags starting with '-l' ('-e' and '-N' do not have this
problem), so there could be false matches.

Change-Id: I827c411de28624019a287f853acc9666e87cbfb9
Reviewed-on: https://go-review.googlesource.com/c/156327


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>