[plt-scheme] Linux x86_64 + JIT bracktraces

For Linux x86_64 users: the current SVN version now supports exception
backtraces (the part after "=== context ===" below) even when the JIT
is enabled.

 Welcome to MzScheme v4.1.3.3 [3m], Copyright (c) 2004-2008 PLT Scheme Inc.
 > (define (f x) ((x)))
 > (f 1)
 procedure application: expected procedure, given: 1 (no arguments)
 
  === context ===
 stdin::1: f
 /Users/mflatt/proj/plt/collects/scheme/private/misc.ss:74:7

Let us know if it doesn't seem to work right, especially as compared to
a 32-bit x86 build.


Thanks to the libunwind team (David Mosberger-Tang et al.) for the
DWARF-2 support.


Matthew