Change directory-specific CFLAGS to use full path

Previously, a specific file or directory could be compiled with custom
CFLAGS by adding a Makefile variable such as:
  CFLAGS_dlmalloc.o = <custom flags for common/dlmalloc.c>
or
  CFLAGS_lib = <custom flags for lib directory>

This method breaks down once multiple files or directories share the
same path.  Eg FLAGS_fileA = <custom flags> would incorrectly result in
both dir1/fileA.c and dir2/fileA.c being compiled with <custom flags>.

This change allows finer grained control which we need once we move
lib_$ARCH to arch/$ARCH/lib/ and lib_generic/ to lib/.  Without this
change all lib/ directories would share the same custom CFLAGS.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>