MFC r306266 (by emaste):

libcompiler_rt: use ${SRCTOP} for the top of the FreeBSD tree

MFC r306377 (by emaste):

libcompiler_rt: move file list to Makefile.inc for reuse elsewhere

Also switch to the style used in the clang390-import branch to reduce
future conflicts.

Reviewed by: dim
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8039