PR go/65349
runtime: Don't call malloc from __go_file_line callback.

When crashing, we call runtime_printcreatedby which calls
__go_file_line which used to call the Go malloc.  If we are
crashing due to a signal due to heap corruption of some sort,
the GO malloc lock might already be held, leading to a crash
within a crash.  Avoid that by assuming that the libbacktrace
strings will stick around, as we already do in go-callers.c.

From-SVN: r221291