linux, linux64: improve SRCS formatting

Sort the entries alphabetically, and list them with one entry per line.
This makes the diffs much cleaner when adding or removing a new entry,
as I will do in the next commit.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 331784880821c6aa55188d0950e98d898c42203c)