tools/nolibc: keep brk(), sbrk(), mmap() away from __sysret()

tools/nolibc: keep brk(), sbrk(), mmap() away from __sysret()

The __sysret() function causes some undesirable casts so we'll revert
it. In order to keep it simple it will now only support integer return
values like in the past, so we must basically revert the changes that
were made to these 3 syscalls which return a pointer so that they
simply rely on their own test and the SET_ERRNO() macro.

Fixes: 4201cfce15fe ("tools/nolibc: clean up sbrk() routine")
Fixes: 924e9539aeaa ("tools/nolibc: clean up mmap() routine")
Fixes: d27447bc2e0a ("tools/nolibc: sys.h: apply __sysret() helper")
Link: https://lore.kernel.org/lkml/20230806095846.GB10627@1wt.eu/
Link: https://lore.kernel.org/lkml/ZNKOJY+g66nkIyvv@1wt.eu/
Cc: Zhangjin Wu <falcon@tinylab.org>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: Thomas Weißschuh <thomas@t-8ch.de>
Signed-off-by: Willy Tarreau <w@1wt.eu>