we currently enable a lot of llvm backends.  i'm sure some of these we don't need.  can we cull these to save space & time ?

$ llc --version
LLVM (http://llvm.org/):
  LLVM version 8.0.0svn
  Optimized build.
  Default target: x86_64-pc-linux-gnu
  Host CPU: skylake-avx512

  Registered Targets:
    aarch64    - AArch64 (little endian)
    aarch64_be - AArch64 (big endian)
    amdgcn     - AMD GCN GPUs
    arm        - ARM
    arm64      - ARM64 (little endian)
    armeb      - ARM (big endian)
    bpf        - BPF (host endian)
    bpfeb      - BPF (big endian)
    bpfel      - BPF (little endian)
    hexagon    - Hexagon                                    <-- kill
    lanai      - Lanai                                      <-- kill
    mips       - Mips                                       <-- kill
    mips64     - Mips64 [experimental]                      <-- kill
    mips64el   - Mips64el [experimental]                    <-- kill
    mipsel     - Mipsel                                     <-- kill
    msp430     - MSP430 [experimental]                      <-- kill
    nvptx      - NVIDIA PTX 32-bit                          <-- kill
    nvptx64    - NVIDIA PTX 64-bit                          <-- kill
    ppc32      - PowerPC 32                                 <-- kill
    ppc64      - PowerPC 64                                 <-- kill
    ppc64le    - PowerPC 64 LE                              <-- kill
    r600       - AMD GPUs HD2XXX-HD6XXX
    sparc      - Sparc                                      <-- kill
    sparcel    - Sparc LE                                   <-- kill
    sparcv9    - Sparc V9                                   <-- kill
    systemz    - SystemZ                                    <-- kill
    thumb      - Thumb
    thumbeb    - Thumb (big endian)
    x86        - 32-bit X86: Pentium-Pro and above
    x86-64     - 64-bit X86: EM64T and AMD64
    xcore      - XCore                                      <-- kill

upstream Gentoo has an LLVM_TARGETS= variable and it looks like our llvm-6 (for boards) checks that.  but we aren't setting it for the SDK, and our custom llvm-8 packages aren't checking it, so i guess we just go ahead and build every target llvm happens support ?