Add mkspecs for the PNaCl-native compilers

These are the native code producing compilers in the
PNaCl toolchain.

Create mkspec naming scheme extending pnacl-newlib-clang:
    pnacl-newlib-clang-x86_32
    pnacl-newlib-clang-x86_64
    pnacl-newlib-clang-arm

Update nacl-configure to accept a pnacl variant
    ./nacl-configure mac_pnacl release x86_64

README.md: Bump required and tested SDK version to
pepper_47. Change build instructions to mention
nacl-x86_64 instead of x86_newlib.

Change-Id: I9854041778b71660b0d9d68396f772dd7b9e4c4b
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>