Update PowerPC thunk detection for PLT stubs

[hainest]

A common idiom in gcc for PLT stubs is

0000000000010000 <0000001e.plt_call.__gmon_start__>:
   10000: 18 00 41 f8  std     r2,24(r1)
   10004: 38 81 82 e9  ld      r12,-32456(r2)
   10008: a6 03 89 7d  mtctr   r12
   1000c: 20 04 80 4e  bctr

With #1929, bctr has correct semantics so these can be detected now.