MFC r312459:

Use SRCTOP-relative paths to other directories instead of .CURDIR-relative ones

This implifies pathing in make/displayed output

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Import libcompiler_rt into HEAD and add Makefiles.

Obtained from: user/ed/compiler-rt