bsd.cpu.mk doesn't have to worry about compilers other than the current
version.
- Allow TARGET_CPUTYPE to override CPUTYPE in bsd.cpu.mk.
- Treat an empty CPUTYPE the same as an undefined CPUTYPE.
- For buildworld, buildkernel, etc., define TARGET_CPUTYPE to CPUTYPE for
native builds and define it to be empty for cross-builds.
TARGET_CPUTYPE is only defined if it is not already defined via the
commandline or environment.
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