[cmake] Remove i686 target that is duplicate to i386
Remove the explicit i686 target that is completely duplicate to
the i386 target, with the latter being used more commonly.
- The runtime built for i686 will be identical to the one built for
- Supporting both -i386 and -i686 suffixes causes unnecessary confusion
on the clang end which has to expect either of them.
- The checks are based on wrong assumption that i686 is defined for
all newer x86 CPUs. In fact, it is only declared when -march=i686 is
explicitly used. It is not available when a more specific (or newer)
-march is used.
Curious enough, if CFLAGS contain -march=i686, the runtime will be built
both for i386 and i686. For any other value, only i386 variant will be
Differential Revision: https://reviews.llvm.org/D26764