tools, perf: Fix symbol offset fallback path in get_srcline

[This is a bug fix over the earlier branch callstack patchkit]

get_srcline has a fallback path when it cannot find the line
number for an address: it returns sym+offset.

The normal line number resolution process uses the al_addr,
which is the address relative to the start of the DSO.
However to compute the offset from the symbol we need the
absolute address.

Since the dso start offset is not easily available here just
pass it in from the caller.

In theory it would be tempting to move the 2objdump conversion
call into get_srcline, but I think that would slow down
annotate quite a bit which caches it, so I didn't do it.

Signed-off-by: Andi Kleen <ak@linux.intel.com>