btrace(8) support to symbolize utrace addresses.

btrace(8) support to symbolize utrace addresses.

This only works for a single static binary where everything was compiled
with -fno-omit-frame-pointer since the stack unwinder requires the
frame-pointer.

A possible btrace script to capture performace of a single process is:
profile:hz:100 / pid == $1 / {
        @[ustack] = count();
}
Then using btrace -p program uprofile.bt `pgrep program` will collect
the information for program.

This is far from perfect but should allow other people to play with this
and hopefully improve work.
OK mpi@