rust: Add libstd linkage mutator for rlibs.

The current state of linkage is that device targets always link
libstd dynamically except for rust_ffi_static which requires a static
libstd linkage. However this prevents producing rust_ffi_static
modules which depend on other Rust libraries as those dependencies
will link libstd dynamically and cause a collision. We also want our
rust_test modules to statically link in libstd as well.

This adds a linkage mutator for rlibs that creates a variant for each
libstd linkage. Dependent modules can then select the variant that
matches their linkage of libstd.

Also fixes an issue where installation paths were being generated for
rlibs and static libs even though they weren't being installed. This broke
when adding the linkage mutator as Make would complain about multiple
targets producing the same output.

Bug: 168729404
Test: rust_ffi_static module with other rustlib dependency can be built.
Change-Id: I955b484bf5809e8fc5517750c7f8df82d3ca8895