sbrk is obsolete and libc only provides it on amd64, arm, i386 and
powerpc. Replace the growing exclusion lists in t_dir.c and t_mlock.c
with a single positive HAS_SBRK definition per file, so architectures
without sbrk need not be enumerated as they are added.
Signed-off-by: Xiaoqiang Zhao <zhaoxiaoqiang007@gmail.com>