[tlbuild] once again test build

[Karl Berry]

It's not worth the (nontrivial) trouble to do the automatic disabling of
luajittex.  Hardwiring the supported cpu list would not be right, and
compiling a test file "at the beginning" is not plausible.

The *alpha* cpu is not supported (and presumably never will be, which is
only reasonable).  That is different than ARM.  Sparc is also not
supported.  If you look at the lj_arch.h source file (or the output from
the build) it is patently obvious.  No need for Luigi to spend time on
it :).

It will be mentioned in a README at some point (I'm working on all that
text now), for what little good it will do.

Thanks to everyone for the build trials.

best,
karl