RFR: 8353174: Clean up thread register handling after 32-bit x86 removal

[Aleksey Shipilev]

Various `MacroAssembler` methods have this code to support passing the thread register, and getting it if `noreg` is passed:


  // determine java_thread register
  if (!java_thread->is_valid()) {
#ifdef _LP64
    java_thread = r15_thread;
#else
    java_thread = rdi;
    get_thread(java_thread);
#endif // LP64
  }


This never happens after 32-bit x86 removal. x86_64 always uses r15_thread. We can clean those up.

These are also the only major users of `MacroAssembler::get_thread` that we want to remove/rename to avoid falling into traps like [JDK-8353176](https://bugs.openjdk.org/browse/JDK-8353176).


Additional testing:
 - [x] Linux x86_64 server fastdebug, `all`

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

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/24323/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24323&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353174
  Stats: 233 lines in 15 files changed: 16 ins; 98 del; 119 mod
  Patch: https://git.openjdk.org/jdk/pull/24323.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24323/head:pull/24323

PR: https://git.openjdk.org/jdk/pull/24323