Set at least PTHREAD_STACK_MIN stack size when creating threads.

This patch makes sure, that the stack size of the new threads are never
less than PTHREAD_STACK_MIN, otherwise the pthread_attr_setstacksize()
function sets the default stack size (8MB), which leads to failure
in the BootUpMemoryUse test.

TEST=cctest/test-mark-compact/BootUpMemoryUse
BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/63183003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17610 ce2b1a6d-e550-0410-aec6-3dcde31c8c00