RFR: Replace BacktraceBuilder and allocation safepoints in async_get_stack_trace

[Coleen Phillimore]

Something like this.  I'm still running tests.

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Commit messages:
 - Fix whitespaces
 - Replace BacktraceBuilder and allocation safepoints in async_get_stack_trace.

Changes: https://git.openjdk.java.net/loom/pull/40/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=40&range=00
  Stats: 79 lines in 1 file changed: 27 ins; 25 del; 27 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