size instead of abusing text symbol

cmd/addr2line needs to know the virtual address of the start
of the text segment (load address plus header size). For
this, it used the text symbol added by the linker. This is
wrong on amd64. Header size is 40 bytes, not 32 like on 386
and arm. Function alignment is 16 bytes causing text to be
at 0x200030.

debug/plan9obj now exports both the load address and the
header size; cmd/addr2line uses this new information and
doesn't rely on text anymore.

LGTM=0intro
R=0intro, gobot, ality
CC=ality, golang-codereviews, jas, mischief
https://golang.org/cl/106460044