bp2build support for host_ldlibs

host_ldlibs are used during linking. Set these in `linkopts` for now.

Note that this CL does not do the `CheckBadHostLdLibs` validation of
Soong. There are some different ways to do this, and these are being
discussed in b/216626461. It is likely that we will need to create a new
property `host_ldlibs` to do the validation. But for now, re-use
`linkopts`.

Bug: 216626461
Test: bp2build unit tests
Change-Id: Id4c77e4460fb1fb003fa58ea27bab5b50ea8cefe