Fix pthread_attr_getstack under valgrind.

valgrind seems to mess with the stack enough that the kernel will
report "[stack:pid]" rather than "[stack]" in /proc/self/maps, so
switch to the task-specific file instead to force "[stack]". (There
are two conditions in the kernel code that decides which form to
output.)

Bug: 17897476

(cherry picked from commit 9afb2f21)

Change-Id: I92c331ef6fb5868af49e75bc595710d290a95f5b