Make tcc work after self-compiling with bounds-check enabled

Make tcc work after self-compiling with bounds-check enabled

For vstack Fabrice used the trick to initialize vtop to &vstack[-1], so
that on first push, vtop becomes &vstack[0] and a value is also stored
there - everything works.

Except that when tcc is compiled with bounds-checking enabled, vstack - 1
returns INVALID_POINTER and oops...

Let's workaround it with artificial 1 vstack slot which will not be
used, but only serve as an indicator that pointing to &vstack[-1] is ok.

Now, tcc, after being self-compiled with -b works:

    $ ./tcc -B. -o tccb  -DONE_SOURCE -DCONFIG_MULTIARCHDIR=\"i386-linux-gnu\" tcc.c  -ldl
    $ cd tests
    $ ../tcc -B.. -run tcctest.c >1
    $ ../tccb -B.. -run tcctest.c >2
    $ diff -u 1 2

and note, tcc's compilation speed is not affected:

    $ ./tcc -B. -bench -DONE_SOURCE -DCONFIG_MULTIARCHDIR=\"i386-linux-gnu\" -c tcc.c

    before: 8270 idents, 47221 lines, 1527730 bytes, 0.152 s, 309800 lines/s, 10.0 MB/s
    after:  8271 idents, 47221 lines, 1527733 bytes, 0.152 s, 310107 lines/s, 10.0 MB/s

But note, that `tcc -b -run tcc` is still broken - for example it crashes
on
    $ cat x.c
    double get100 () { return 100.0; }

    $ ./tcc -B. -b -DTCC_TARGET_I386 -DCONFIG_MULTIARCHDIR=\"i386-linux-gnu\"  -run   \
        -DONE_SOURCE ./tcc.c -B. -c x.c
    Runtime error: dereferencing invalid pointer
    ./tccpp.c:1953: at 0xa7beebdf parse_number() (included from ./libtcc.c, ./tcc.c)
    ./tccpp.c:3003: by 0xa7bf0708 next() (included from ./libtcc.c, ./tcc.c)
    ./tccgen.c:4465: by 0xa7bfe348 block() (included from ./libtcc.c, ./tcc.c)
    ./tccgen.c:4440: by 0xa7bfe212 block() (included from ./libtcc.c, ./tcc.c)
    ./tccgen.c:5529: by 0xa7c01929 gen_function() (included from ./libtcc.c, ./tcc.c)
    ./tccgen.c:5767: by 0xa7c02602 decl0() (included from ./libtcc.c, ./tcc.c)

that's because lib/bcheck.c runtime needs more fixes -- see next
patches.