The pcln file number was being encoded incorrectly. The recorded delta was always against -1, not against the previous value.

Update #7369

This CL fixes the bad DWARF file numbers. It does not, however, fix the gdb continue-to-end bug.

LGTM=iant
R=rsc, minux.ma, iant
CC=golang-codereviews, graham
https://golang.org/cl/68960046