style: Add shared gem5 headers to the style checker

style: Add shared gem5 headers to the style checker

Teach the style checker about common headers living in gem5/. These
should be included after any global library headers (e.g., C headers
or STL headers), but before the normal gem5 headers.

Change-Id: I322f841420e361c16314be8fa4cbd1e86d2bfa9f
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/4300
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>