Add initial support for Thumb for ARMv7a

    
    Add support for the R_ARM_THM relocations used in the objects present
    in arm-linux-gnueabihf-gcc. These are:
    R_ARM_THM_CALL
    R_ARM_THM_JUMP11
    R_ARM_THM_JUMP19
    R_ARM_THM_JUMP24
    R_ARM_THM_MOVT_ABS
    R_ARM_THM_MOVW_ABS_NC
    
    Interworking between ARM and Thumb is partially supported with BLX.
    The R_ARM_CALL relocation for ARM instructions and R_ARM_THM_CALL
    relocation for Thumb instructions will write out a BL or BLX depending
    on the state of the Target.
    
    Assumptions:
    - Availability of BLX and extended range of Thumb 4-byte Branch
      instructions.
    - In relocateOne if (Val & 0x1) == 1 target is Thumb, 0 is ARM.
      This will hold for objects that comply with the ABI for the
      ARM architecture.
    
    This is sufficient for hello world to work with a recent
    arm-linux-gnueabihf distribution.
    
    Limitations:
    No interworking for R_ARM_JUMP24, R_ARM_THM_JUMP24, R_ARM_THM_JUMP19
    and the deprecated R_ARM_PLT32 and R_ARM_PC24 instructions as these
    cannot be written out as a BLX and need a state change thunk.
    
    No range extension thunks. The R_ARM_JUMP24 and R_ARM_THM_CALL have a
    range of 16Mb

llvm-svn: 272881