[ARM] Implementation of PLT: handling of IFUNC calls (gnu_indirect_function)

This diff includes implementation of linking calls to ifunc functions.
It provides ifunc entries in PLT and corresponding relocations (R_ARM_ALU_PC_G0_NC,
R_ARM_ALU_PC_G1_NC, R_ARM_LDR_PC_G2 for link-time and R_ARM_IRELATIVE for run-time).

Differential Revision: http://reviews.llvm.org/D7833

llvm-svn: 233277