cmd/ld: fix PC deltas in DWARF line number table
cmd/ld: fix PC deltas in DWARF line number table

The putpclcdelta function set the DWARF line number PC to
s->value + pcline->pc, which is correct, but the code then set
the local variable pc to epc, which can be a different value.
This caused the next delta in the DWARF table to be wrong.

Fixes issue 8098.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/104950045

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