Fix for pr24346: arm asm label calculation error in sub

Some ARM instructions encode 32-bit immediates as a 8-bit integer (0-255)
and a 4-bit rotation (0-30, even) in its least significant 12 bits. The
original fixup, FK_Data_4, patches the instruction by the value bit-to-bit,
regardless of the encoding. For example, assuming the label L1 and L2 are
0x0 and 0x104 respectively, the following instruction:

  add r0, r0, #(L2 - L1) ; expects 0x104, i.e., 260

would be assembled to the following, which adds 1 to r0, instead of 260:

  e2800104 add r0, r0, #4, 2 ; equivalently 1

The new fixup kind fixup_arm_mod_imm takes care of the encoding:

  e2800f41 add r0, r0, #260

Patch by Ting-Yuan Huang!

llvm-svn: 265122