tools/mm: allow users to provide additional cflags/ldflags

tools/mm: allow users to provide additional cflags/ldflags

Right now there is no way to provide additional cflags/ldflags when
building tools/vm binaries.  And using eg.  make CFLAGS=<options> will
override the CFLAGS being set in the Makefile, making the build fail since
it requires the include of the ../lib dir (for libapi).

This change then allows you to specify:
CFLAGS=<options> LDFLAGS=<options> make V=1 -C tools/vm

And the options will be correctly appended as can be seen from the
make output.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Herton R. Krzesinski <[email protected]>
Cc: Don Zickus <[email protected]>
Cc: Justin Forbes <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Scott Weaver <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>