RFR: Replace BacktraceBuilder and allocation safepoints in async_get_stack_trace [v2]

[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:

  Cosmetic cleanups.

-------------

Changes:
  - all: https://git.openjdk.java.net/loom/pull/40/files
  - new: https://git.openjdk.java.net/loom/pull/40/files/6594c8ef..50e516c1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=40&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=40&range=00-01

  Stats: 10 lines in 1 file changed: 3 ins; 5 del; 2 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