*Actually* fix `make install` with rlibs

Turns out that we only want to install the target rlibs, not the host rlibs.
I had it backwards the first time, then mixed up the second time, but this time
should get it right.

There's no need for host rlib files because none of them are needed at runtime.
auto merge of #10753 : alexcrichton/rust/fix-make-install, r=huonw

Turns out that we only want to install the target rlibs, not the host rlibs.
I had it backwards the first time, then mixed up the second time, but this time
should get it right.

There's no need for host rlib files because none of them are needed at runtime.