By default, when 32 bit x86 build is requested, i686 instruction set is chosen now. i386 instruction set removed since de facto it is not supported by some core libraries (Boost.Atomic and Boost.SmartPtr) and major operating systems. The minimum 32 bit x86 instruction set is i486 now. Also added a few more instruction sets that appeared in recent CPUs.

svn path=/trunk/tools/build/; revision=84221