librustllvm.so missing from prefix target lib directory #6247

Running a "make install" vs master/incoming will install librustllvm.so to the host prefix library directory, but not the target library directory. In my case, this prevented me from linking a valid Rust program against librustdoc because rustc was unable to find librustllvm.so.

The fix is a trivial change to mk/install.mk: add the following line to the rule for install-target-$(1)-host-$(2): ..., after the equivalent line for CFG_RUNTIME_$(1):

$$(Q)$$(call INSTALL_LIB,$$(TL$(1)$(2)),$$(PTL$(1)$(2)),$$(CFG_RUSTLLVM_$(1)))

On 64-bit Linux, this will drop librustllvm.so in $prefix/lib/rustc/x86_64-unknown-linux-gnu/lib along with everything else.

Happy to provide a pull request, but this appears to be a one-liner.