MFC r305033,r305041,r305170:

MFC r305033,r305041,r305170:

r305033:

Minor Makefile simplifications for lib/atf/...

- Replace uses of `${.CURDIR:H:H:H}` with ${SRCTOP}
- Use built-in :H operator instead of ".." when enumerating paths.

r305041:

Filter certain compile-time options into -DATF_BUILD_*

Items filtered through are:

- Constant defines (-D)
- Include flags (-I)
- Linker flags (-L)
- Optimization level (-O)
- Warnings / linker flags (-W)
- Preprocessor options (-f)

This fixes the scenario hit by the Jenkins job where it's infecting
the build with --sysroot, etc options from the Jenkins build in the
tests.

Prefix all intermediate variables (_CFLAGS, etc) with "ATF_BUILD" [*].

Requested by:	jmmv

r305170:

Don't bake all of CC/CPP/CXX into CFLAGS

Capture executable names for CC, CPP, CXX (assumed to be the
first non-CCACHE_BIN word).

This change strips out all of the cross-compiler arguments, (-target,
-B, etc), added to ${CC}, etc via ${CROSSENV} in Makefile.inc1, so it
doesn't infect the build and subsequently the test.

Add comments noting why this logic is being added, and why the logic in
r305041 was necessary/what it was trying to achieve.

This is required after recent changes made to the toolchain to always
specify --sysroot, -target, -B, etc with clang in buildworld (presumably
r304681).