Allow specifying tool path/prefix for tests with CROSS_COMPILE.

CROSS_COMPILE environment variable is a common convention to specify the path
and/or prefix to cross compilation tools (e.g. ar, objcopy).

Test Plan:
dotest.py $DOTEST_OPTS -t -p 'TestBSDArchives.py|TestBreakpointCommandsFromPython.py|TestFormats.py|TestObjCDynamicValue.py'
All of these tests now compile successfully and pass running macosx -> linux
using a cross compilation toolchain prefixed by CROSS_COMPILE without requiring
changing your PATH. They still pass when run locally on macosx.

Differential Revision: http://reviews.llvm.org/D9072

llvm-svn: 235320