RFR: 8286897: Loom: Cleanup x86_64 StubGenerator [v3]

[Aleksey Shipilev]

> During x86_32 porting, I ended up rewriting the copy of x86_64 StubGenerator significantly to make it more streamlined. The original x86_64 StubGenerator should enjoy the similar streamlining, since it would likely to be the template for many other architectures. 
> 
> There should be no behavior changes, except one small one. For some weird reason, the `__ movptr(c_rarg1, rsp);` in `generate_cont_doYield` used to be much earlier, between `start` and `the_pc`. I don't believe that's part of the frame setup. It looks like a copy-paste glitch from `generate_throw_exception`. So I just moved it here. No tests complain about this move.
> 
> The rest of the changes themselves should speak for themselves, but here is a brief list:
>  - no need for `assert_asm` macros, instead do the inline checks, like the rest of Hotspot and Loom code
>  - `_masm` and `masm` shadowing can just use whatever is currently defined as `__`
>  - more strong cohesion between `post_call_nop`, its associated PC and its oopmap
>  - consistently use `call_VM_leaf` that only takes the number of arguments, as we are doing argument pushes ourselves anyway;
>  - for many moves, it is easier to use `ptr` flavors (e.g. `testq` -> `testptr`) to ease copy-pasting to other arches;
>  - `generate_cont_thaw` should just do the `enabled()` check and carry the `StubCodeMark`, like other stubs do;
>  - inline comments are reflowed to block-like comments, matching the usual style of stub code;
>  - labels are prefixed with `L_` for clarity (used in some places in Hotspot);
>  - `jfr_prolog` and `jfr_epilog` are used only once and do not carry their weight;
>  - `continuation_*` are documented a bit better;
> 
> Additional testing:
>  - [x] Linux x86_64 fastdebug, `java/lang/Thread/virtual`
>  - [x] Linux x86_64 fastdebug, `serviceability/jvmti`
>  - [x] Linux x86_64 fastdebug, `runtime/vthread`

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

  Review feedback

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8755/files
  - new: https://git.openjdk.java.net/jdk/pull/8755/files/b4a29e72..8a840a81

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8755&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8755&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8755.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8755/head:pull/8755

PR: https://git.openjdk.java.net/jdk/pull/8755