cmd/compile: add new symbol for debug line numbers

This is broken out from: CL 187117

This new symbol will be populated by the compiler and contain debug line
information that's currently generated in the linker. One might say it's
sad to create a new symbol, but this symbol will replace the isStmt
symbols.

Testing: Ran go build -toolexec 'toolstash -cmp'

Change-Id: If8f7ae4b43b7247076605b6429b7d03a1fd239c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/188238


Reviewed-by: Austin Clements <austin@google.com>