More PNaCL fixes (without GYP/Makefile tweaks)

This is basically https://codereview.chromium.org/513923005/ with a
few changes:

   * Makefile.nacl and v8.gyp are untouched.

   * MAP_NORESERVE-handling is more defensive.

   * Added ugly busy-wait emulation of sem_timedwait.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/521473003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23513 ce2b1a6d-e550-0410-aec6-3dcde31c8c00