Have pthread_attr_getstack for the main thread report RLIMIT_STACK...

...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 9e4ffa70)

Change-Id: I73f219a569917b2e4546c09436d7ef5231facc07