x86/cpu/AMD: Add speculative control support for AMD

x86/cpu/AMD: Add speculative control support for AMD

CVE-2017-5753
CVE-2017-5715

Add speculative control support for AMD processors. For AMD, speculative
control is indicated as follows:

  CPUID EAX=0x00000007, ECX=0x00 return EDX[26] indicates support for
  both IBRS and IBPB.

  CPUID EAX=0x80000008, ECX=0x00 return EBX[12] indicates support for
  just IBPB.

On AMD family 0x10, 0x12 and 0x16 processors where either of the above
features are not supported, IBPB can be achieved by disabling
indirect branch predictor support in MSR 0xc0011021[14] at boot.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>