The linker always uses .plt for externals, so libcFunc is now an actual
external symbol instead of a pointer to one.

Fixes most of the breakage introduced in previous CL.

Change-Id: I64b8c96f93127f2d13b5289b024677fd3ea7dbea
Reviewed-on: https://go-review.googlesource.com/8215Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>