cmd/link: let writelines work when pcstmt symbol doesn't exist
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 <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>

golang/go,golang/go,CAFxX/go,cloudflare/go,golang/go,josharian/go,akutz/go,akutz/go,cloudflare/go,nawawi/go,akutz/go,golang/go,nawawi/go,akutz/go,CAFxX/go,CAFxX/go,cloudflare/go,akutz/go,nawawi/go,josharian/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go,golang/go,nawawi/go,akutz/go,CAFxX/go,josharian/go,CAFxX/go,akutz/go,cloudflare/go,cloudflare/go,nawawi/go,akutz/go,josharian/go,nawawi/go,josharian/go,nawawi/go,golang/go,josharian/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,josharian/go,golang/go,cloudflare/go