[x86 setup] Make struct ist_info cross-architecture, and use in setup code

Make "struct ist_info" valid on both i386 and x86-64, and use the
structure by name in the setup code.  Additionally, "Intel SpeedStep
IST" is redundant, refer to it as IST consistently.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>