arch.7: move discontinued CPU archs to own table

[emaste]

Summary

With MIPS' retirement we have more discontinued architectures than supported, making the table somewhat unclear. Split the table in two.

[emaste]

FreeBSD architecture support varies by release.  This table shows
currently supported CPU architectures along with the first FreeBSD
release to support each architecture.

      Architecture    Initial Release
      aarch64         11.0
      amd64           5.1
      armv6           10.0
      armv7           12.0
      i386            1.0
      powerpc         6.0
      powerpcspe      12.0
      powerpc64       9.0
      powerpc64le     13.0
      riscv64         12.0
      riscv64sf       12.0

Discontinued architectures are shown in the following table.

      Architecture    Initial Release    Final Release
      alpha           3.2                6.4
      arm             6.0                12.x
      armeb           8.0                11.4
      ia64            5.0                10.4
      mips            8.0                13.x
      mipsel          9.0                13.x
      mipselhf        12.0               13.x
      mipshf          12.0               13.x
      mipsn32         9.0                13.x
      mips64          9.0                13.x
      mips64el        9.0                13.x
      mips64elhf      12.0               13.x
      mips64hf        12.0               13.x
      pc98            2.2                11.4
      sparc64         5.0                12.x

[allanjude]

Nice improvement

reviewed by: allanjude