allow custom PREFIX and MULTILIB in bulding TvmJIT

`make TVMJIT_PREFIX=/usr` will pass PREFIX to TvmJIT Makefile.

`make TVMJIT_MULTILIB="lib/$(gcc -print-multiarch)"` will pass MULTILIB to
TvmJIT Makefile.

It's useful for building Shine with default package.path and package.cpath
values mimicking the same default values of LuaJIT installed via way preferred
by Linux distro (like LuaJIT packaged in Debian and installed via `apt`).
document TVMJIT_PREFIX and TVMJIT_MULTILIB
Merge pull request #66 from leonidborisenko/custom-tvmjit-prefix-and-multilib

Allow custom PREFIX and MULTILIB in building TvmJIT