Make FirstTarget treat HostCross separately from Host

Use Target.Os and Target.HostCross as the key in FirstTarget so that
it returns a separate target for host and host cross architectures.
This is useful when host and host cross are both linux_musl, but
host cross is an independenct architecture like arm64.

Bug: 236052820
Test: cross compile linux_musl-arm64
Change-Id: If75790001afe9d0f9d4d8166f207847851812297