*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.
remove useless `transmute_immut` function
rustc: filter out empty linker args

This is inspired by a mystifying linker failure when using `pkg-config` to
generate the linker args: `pkg-config` produces output that ends in a
space, thus resulting in an empty linker argument.

Also added some updates to the concerning error messages that helped
spotting this bug.
auto merge of #10749 : Blei/rust/fix-linker-args, r=alexcrichton

This is inspired by a mystifying linker failure when using `pkg-config` to
generate the linker args: `pkg-config` produces output that ends in a
space, thus resulting in an empty linker argument.

Also added some updates to the concerning error messages that helped
spotting this bug.
auto merge of #10756 : thestinger/rust/transmute, r=alexcrichton
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.