cmd/link: let writelines work when pcstmt symbol doesn't exist

The assembler does not produce pcstmt symbols, writeline should be able
to work even if no pcstmt symbol exists for a given function.

Fixes #25216, #25191

Change-Id: I41e16df1e7c8ca59d27e7514537609e309a51c51
Reviewed-on: https://go-review.googlesource.com/110816


Reviewed-by: David Chase <drchase@google.com>