tools/nolibc: clean up sbrk() routine

Fix up the error reported by scripts/checkpatch.pl:

    ERROR: do not use assignment in if condition
    #95: FILE: tools/include/nolibc/sys.h:95:
    +	if ((ret = sys_brk(0)) && (sys_brk(ret + inc) == ret + inc))

Apply the new generic __sysret() to merge the SET_ERRNO() and return
lines.

Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>