When building on i386, both clang and gcc define a builtin 'i386' macro (see
discussion in https://github.com/llvm/llvm-project/pull/137063). This causes
build errors in the JITLink/i386 backend when attempting to build LLVM on i386.
This commit renames the 'i386' backend (namespaces, APIs and files) to 'x86' to
avoid this issue.
