test: Add the ability to force a host target

The new macro loading infrastructure needs the ability to force a
procedural-macro crate to be built with the host architecture rather than the
target architecture (because the compiler is just about to dlopen it).
auto merge of #11618 : alexcrichton/rust/force-host, r=brson

The new macro loading infrastructure needs the ability to force a
procedural-macro crate to be built with the host architecture rather than the
target architecture (because the compiler is just about to dlopen it).