...rather than just what's already mapped in. This seems somewhat
contrary to POSIX's "All pages within the stack described by stackaddr
and stacksize shall be both readable and writable by the thread", but
it's what glibc does.
Bug: 17111575
(cherry picked from commit 9e4ffa7032eaab308876b8e3da86b05c3c613878)
Change-Id: I73f219a569917b2e4546c09436d7ef5231facc07