Fix our x86 PIC_PROLOGUE.

The old definition only worked for functions that didn't use numbered
local labels. Upstream uses '666' not only as some kind of BSD in-joke,
but also because there's little likelihood of any function having
labels that high.

There's a wider question about whether we actually want to go via the
PLT at all in this code, but that's a question for another day.

(cherry-pick of 72d7e667.)

Bug: 16906712
Change-Id: I3cd8ecc448b33f942bb6e783931808ef39091489