runtime: fix VDSO lookup to use dynamic hash table
runtime: fix VDSO lookup to use dynamic hash table

Reportedly in the Linux 3.16 kernel the VDSO will not have
section headers or a normal symbol table.

Too late for 1.3 but perhaps for 1.3.1, if there is one.

Fixes issue 8197.

LGTM=rsc
R=golang-codereviews, mattn.jp, rsc
CC=golang-codereviews
https://codereview.appspot.com/101260044

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang