libgo: reduce overhead for memory/block/mutex profiling

libgo: reduce overhead for memory/block/mutex profiling

    Revise the gccgo version of memory/block/mutex profiling to reduce
    runtime overhead. The main change is to collect raw stack traces while
    the profile is on line, then post-process the stacks just prior to the
    point where we are ready to use the final product. Memory profiling
    (at a very low sampling rate) is enabled by default, and the overhead
    of the symbolization / DWARF-reading from backtrace_full was slowing
    things down relative to the main Go runtime.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/171497

From-SVN: r271172