Initial support for ARM in lld.

    
    Add support for an ARM Target and the initial set of relocations
    and PLT entries that are necessary for an ARM only hello world to
    link. This has been tested against an ARM only sysroot from the
    4.2.0 CodeSourcery Lite release.
    
    Tests have been added to test/ELF for the support that has been
    implemented.
    
    Main limitations:
    - No Thumb support
    - Relocations incomplete
    - No C++ exceptions support
    - No TLS support
    - No range extension or interworking veneer (thunk) support
    - No Build Attribute support
    - No Big-endian support
    
    The deprecated relocations R_ARM_PLT32 and R_ARM_PC24 have been
    implemented as these are used by the 4.2.0 CodeSourcery Lite release.

llvm-svn: 271993