Add makefile debugging rule for printing out the value of a variable.

From http://blog.melski.net/tag/debugging-makefiles/.

Example:

[13:14:59] johnny:/Volumes/data/lldb/svn/trunk/test/class_static $ make print-CC
CC=gcc
  origin = file
  flavor = recursive
   value = gcc
[13:15:09] johnny:/Volumes/data/lldb/svn/trunk/test/class_static $ make print-LD
LD=  g++
  origin = file
  flavor = recursive
   value = $(call cxx_linker,$(CC))
[13:15:21] johnny:/Volumes/data/lldb/svn/trunk/test/class_static $ make print-CXX
CXX=  g++
  origin = file
  flavor = recursive
   value = $(call cxx_compiler,$(CC))
[13:15:29] johnny:/Volumes/data/lldb/svn/trunk/test/class_static $

llvm-svn: 123469