Add linker support for ELF on ARM

Add support for ELF objects on ARM to the runtime linker. While the list of
relocation types in the "ELF for the ARM Architecture" specification spans four
pages, we thankfully only see a handful of these in the wild. Thus, at the
moment we only support the following .rel relocation types,

 * R_ARM_ABS32
 * R_ARM_TARGET1
 * R_ARM_REL32
 * R_ARM_CALL
 * R_ARM_JUMP24
 * R_ARM_MOVT_ABS
 * R_ARM_MOVW_ABS_NC
 * R_ARM_THM_CALL
 * R_ARM_THM_JUMP24
 * R_ARM_THM_MOVT_ABS
 * R_ARM_THM_MOVW_ABS_NC
 * R_ARM_THM_JUMP8
 * R_ARM_THM_JUMP11

Signed-off-by: Ben Gamari <bgamari.foss@gmail.com>