Generate PLT thumb stub only when necessary

Generate PLT thumb stub only when necessary

Generate PLT thumb stub for an ARM PLT entry only when at least one
Thumb instruction branches to that entry.

Warning: To save space, this commit reuses the bit 0 of entries of
got_offsets array. The GOT offset is thus saved in a 31 bit value.
Make sure to divide by 2 (right shift by 1) an offset before storing it
there and conversely to multiply the value by 2 (left shift by 1) before
using it.