[build] Inhibit spurious array bounds warning on some versions of gcc

[build] Inhibit spurious array bounds warning on some versions of gcc

Some versions of gcc (observed with gcc 9.3.0 on NixOS Linux) produce
a spurious warning about an out-of-bounds array access for the
isa_extra_probe_addrs[] array.

Work around this compiler bug by redefining the array index as a
signed long, which seems to somehow avoid this spurious warning.

Debugged-by: Manuel Mendez <mmendez534@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>