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.

Also filter the targets returned by ctx.MultiTargets() to match
the HostCross value of ctx.Target() to prevent the newly created
HostCross variants from colliding with Host variants in JNI or
test data attached to Java targets using a common arch.

This relands If75790001afe9d0f9d4d8166f207847851812297 with the
addition of the ctx.MultiTargets() filtering.

Bug: 236052820
Test: TestArchMutator
Change-Id: Ia6fe1185915d174d0ad6b401c227e0e57bee5c24