runtime.stackBarrier is a strange function: it is only ever "called" by
smashing its address into a LR slot on the stack. Calling it like this
certainly does not adhere to the rule that r12 is set to the global entry point
before calling it and the prologue instrutions that compute r2 from r12 in fact
just corrupt r2, which is bad because the function that stackBarrier returns to
probably uses r2 to access global data.

Fortunately stackBarrier itself does not access any global data and so does not
depend on the value of r2, meaning we can ignore the ABI rules and simply skip
inserting the prologue instructions into this specific function.

Fixes 64bit.go, append.go and fixedbugs/issue13169.go from "cd test; go run
run.go -linkshared".

Change-Id: I606864133a83935899398e2d42edd08a946aab24
Reviewed-on: https://go-review.googlesource.com/17281Reviewed-by: Austin Clements <austin@google.com>