RFR: 8286808: Loom: Simplify generate_cont_thaw by passing thaw_kind directly [v2]

[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. Unused x86_32 and PPC64 stubs are just deleted without refactoring the args; this is supposed to produce a cleaner merge problem for current porting work.
> 
> Additional testing:
>  - [x] `java/lang/Thread/virtual` on Linux x86_64 fastdebug
>  - [x] `java/lang/Thread/virtual` on Linux AArch64 fastdebug
>  - [x] Build Linux x86_32
>  - [x] Build Linux PPC64

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Remove unused generate_cont_thaw stubs too

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8725/files
  - new: https://git.openjdk.java.net/jdk/pull/8725/files/0d986818..d97dab48

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8725&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8725&range=00-01

  Stats: 12 lines in 2 files changed: 0 ins; 12 del; 0 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