RFR: 8286808: Loom: Simplify generate_cont_thaw by passing thaw_kind directly

[Aleksey Shipilev]

I was staring at this code for x86_32 port, and it seems `exception` implies `return_barrier`. This mostly irks me because we are passing `2` to `Continuation::prepare_thaw` and implicitly coercing it to `bool` during the call. We can instead call with `1` when `return_barrier`.

Further cleanup makes the change more obvious; we can pass `thaw_kind` directly to `generate_cont_thaw`, and thus simplify the code. 

Additional testing:
 - [x] `java/lang/Thread/virtual` on Linux x86_64 fastdebug
 - [x] `java/lang/Thread/virtual` on Linux AArch64 fastdebug

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Commit messages:
 - Fix AArch64
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/8725/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8725&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286808
  Stats: 33 lines in 3 files changed: 12 ins; 4 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8725.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8725/head:pull/8725

PR: https://git.openjdk.java.net/jdk/pull/8725