Don't clobber CFLAGS_EXTRAS in tests.

To run tests against a different target platform many extra compiler flags are
needed to specify sysroot, include dirs, etc. The environment variable
CFLAGS_EXTRAS seems suited for this purpose except that several Makefiles
clobber the current flags. This change modifies all of these to add to
CFLAGS_EXTRAS instead.

Test Plan:
Verify no regressions in ninja check-lldb.
Run tests using CFLAGS_EXTRAS to specify cross compilation flags for a different
target running lldb-server platform.

Differential Revision: http://reviews.llvm.org/D8559

llvm-svn: 233066