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 <emm.odeke@gmail.com>