Specify SONAME when building Rust shared libs.

Without specifying -soname, cc binaries are unable to locate Rust
provided shared libraries. This adds the -soname linker flag for shared
libraries.

Bug: 158490355
Test: readelf -d <cc_binary> shows the expected SONAME
Change-Id: I66852a7ce24d5ea5e426f11bc1834fb56a150628