Powerpc: Fix checking for scv support, add check to scv instruction parsing.

Powerpc: Fix checking for scv support, add check to scv instruction parsing.

The check for the scv instruction in coregrind/m_machine.c issues an scv
instruction and uses sigill to determine if the instruction is supported.
Issuing scv on systems that don't support scv, i.e. scv support is not in
HWCAPS2, generates a message in dmesg "Facility 'SCV' unavailable (12),
exception".

This patch removes the sigill based scv instruction test from
coregrind/m_machine.c.  The scv support is now determined by reading the
HWCAPS2 in setup_client_stack().  VG_(machine_ppc64_set_scv_support) is
called to set the flag ppc_scv_supported in struct VexArchInfo.

The allow_scv flag is added in disInstr_PPC_WRK.  The allow_scv flag is
used to ensure the host has support for scv before generating the iops for
the scv instruction.