RFR: Replace BacktraceBuilder and allocation safepoints in async_get_stack_trace [v4]

[Coleen Phillimore]

> Something like this.  I'm still running tests.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Remove 2 stack trace tests that now pass.

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Changes:
  - all: https://git.openjdk.java.net/loom/pull/40/files
  - new: https://git.openjdk.java.net/loom/pull/40/files/dbedc029..7c2574ca

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=40&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=40&range=02-03

  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/loom/pull/40.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/40/head:pull/40

PR: https://git.openjdk.java.net/loom/pull/40