[syslinux] New __lowmem declaration

[H. Peter Anvin]

I decided that we really needed the ability to allocate a lowmem buffer
(a buffer in the low 1 MB, so we can pass the address to SEG() and
OFFS()) in core C code.  It might not be necessary in the long run, but
in the short term it should help development along.

To use it, #include "core.h" and declare a buffer with __lowmem:

static __lowmem char buffer[4096];

Right now the total amount of __lowmem is limited to little over 100K;
this is enough to put the cache buffer in (in which case we can
immediately remove the cache_buf_seg).  In the long run, we want to
minimize this as much as is practical, although in the famous words of
Albert Einstein:

	"Make everything as simple as possible, but no simpler."

	-hpa