[docs] Use x86_64 and i386 instead of x86 as arch for triples.

Summary: x86 is not a valid arch for target triples, but x86_64 and i386 are.

Reviewers: rengolin, silvas

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D26960

llvm-svn: 288723