powerpc64: Add the DSCR facility on POWER8 and later

The Data Stream Control Register (DSCR) is privileged on POWER7, but
unprivileged (different register) on POWER8 and later. However, it's now
guarded by a new register, the Facility Status and Control Register, instead of
the MSR like other pre-existing facilities (FPU, Altivec). The FSCR must be
managed explicitly, since it's effectively an extension of the MSR.

Tested by: Brandon Bergren