Pad Go symbol table out to page boundary when linking dynamically.

Pad Go symbol table out to page boundary when linking dynamically.

This avoids a crash when using cgo where glibc's malloc thinks
that it can use some of the memory following the symbol table.
This fails because the symbol table is mapped read-only, which
affects the whole page.

R=rsc
CC=golang-dev
https://golang.org/cl/1616042