rustc/rustpkg: Use a target-specific subdirectory in build/ and lib/

As per rustpkg.md, rustpkg now builds in a target-specific
subdirectory of build/, and installs libraries into a target-specific
subdirectory of lib.

Closes #8672
auto merge of #9151 : catamorphism/rust/rustpkg-target-specific-subdirectory, r=brson

r? @brson As per rustpkg.md, rustpkg now builds in a target-specific
subdirectory of build/, and installs libraries into a target-specific
subdirectory of lib.

Closes #8672