Kbuild: link against libc only for newlib

Right now we are building minimal libc as an archive (lib.a)
and do not need to link against -lc -lm in that case.
Link against libc/libm only when using an external libc.

Change-Id: I1af0eaf5c059bd5f0c8c16cda28114c6dc10b156
Signed-off-by: Anas Nashif <anas.nashif@intel.com>