* libbacktrace support
* switch to a generic stack trace overriding mechanism
When "nimStackTraceOverride" is defined, once of the imported modules
can register its own procedure to replace the default stack trace
generation by calling `registerStackTraceOverride(myOwnProc)`.
Tested with `./koch boot -d:release --debugger:native -d:nimStackTraceOverride --import:libbacktrace`
for the compiler itself and `./bin/nim c -r -f --stacktrace:off --debugger:native -d:nimStackTraceOverride --import:libbacktrace foo.nim`
for an external program.
* make the StackTraceOverrideProc {.noinline.}
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