cmd/link: put C static symbols in the symbol table

We don't put Go static symbols in the symbol table, as they are
always compiler-generated (there is no way to define a static
symbol in user code in Go). We retain static symbols in assembly
code, as it may be user-defined. Also retain static symbols in C.

Change-Id: Ie0425bb4df33f183857b1fd5ba4b2bdfdc497571
Reviewed-on: https://go-review.googlesource.com/c/go/+/263259


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>