cmd/compile/internal/syntax: add -skip flag to exclude files from TestStdLib
cmd/compile/internal/syntax: add -skip flag to exclude files from TestStdLib

TestStdLib reports parsed lines and lines/s information. To make
it easier to compare apples to apples when making changes in the
std lib, a regular expression provided via the -skip flag filters
files we don't want to process.

Change-Id: I27d9c32032eac4e78581205892e4f26947c91bd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/221600
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>

CAFxX/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,akutz/go,akutz/go,cloudflare/go,akutz/go,akutz/go,akutz/go,cloudflare/go,cloudflare/go,akutz/go,CAFxX/go,golang/go,nawawi/go,golang/go,golang/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,akutz/go,golang/go,golang/go,golang/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,nawawi/go