Add backtrace to exception output.

Add backtrace to exception output.

On Xtensa windowed ABI backtrace is easy to do, and it's immensely
useful for debugging, so - do it by default.
We try to be careful and not deref bogus pointers while walking the
frames.

Example output (from debugging espressif/esp-idf#133):

Backtrace: 0x1:0x3ffc51e0 0x400e9dfa:0x3ffc5210 0x400e9ebc:0x3ffc5230 0x400ec487:0x3ffc5260

With just addr2line utility from binutils, this already pinpoints the
location of the crash.