use of TOK_alloca breaks cross compiler build

use of TOK_alloca breaks cross compiler build

VLA inserts a call to alloca via enum TOK_alloca, but TOK_alloca
only exists on I386 and X86_64 targets.  This patch just emits an
error at compile-time if someone tries to compile some VLA code
for a TOK_alloca-less target. The best solution might be to just
push the problem to link-time, since the existence-or-not of a
alloca implementation can only be determined by linking.  It seems
like just declaring TOK_alloca unconditionally would achieve that,
but for now, this at least gets the cross compilers to build.