x86/e820: fix build with gcc9

x86/e820: fix build with gcc9

e820.c: In function ‘clip_to_limit’:
.../xen/include/asm/string.h:10:26: error: ‘__builtin_memmove’ offset [-16, -36] is out of the bounds [0, 20484] of object ‘e820’ with type ‘struct e820map’ [-Werror=array-bounds]
   10 | #define memmove(d, s, n) __builtin_memmove(d, s, n)
      |                          ^~~~~~~~~~~~~~~~~~~~~~~~~~
e820.c:404:13: note: in expansion of macro ‘memmove’
  404 |             memmove(&e820.map[i], &e820.map[i+1],
      |             ^~~~~~~
e820.c:36:16: note: ‘e820’ declared here
   36 | struct e820map e820;
      |                ^~~~

While I can't see where the negative offsets would come from, converting
the loop index to unsigned type helps. Take the opportunity and also
convert several other local variables and copy_e820_map()'s second
parameter to unsigned int (and bool in one case).

Reported-by: Charles Arnold <carnold@suse.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>