Add backtrace generation to MALLOC_DEBUG memory corruption reports

This patch allows astmm to access the backtrace generation code in Asterisk.
When memory is allocated, a backtrace is created and stored with the memory
region that tracks the allocation. If a memory corruption is detected, the
backtrace is printed to the astmm log. The backtrace will make use of the
BETTER_BACKTRACES build option if available.

As a result, this patch moves the backtrace generation code into its own file
and uses the non-wrapped versions of the C library memory allocation routines.
This allows the memory allocation code to safely use the backtrace generation
routines without infinitely recursing.

Review: https://reviewboard.asterisk.org/r/2567


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@391012 f38db490-d61c-443f-a65b-d21fe96a405b