RFR: 8298241: Replace C-style casts with JavaThread::cast

[David Holmes]

This is a simple cleanup RFE to get rid of old-style C casts in relation to JavaThread.

In many cases involving NULL/nullptr the cast could just be dropped. Sometimes a static cast is needed to disambiguate overloads.

A couple of reinterpret_cast are needed when doing int/ptr conversion.

static_cast is used for void* conversion.

The other changes should be self explanatory.

The changes in

src/hotspot/os_cpu/bsd_aarch64/javaThread_bsd_aarch64.cpp
src/hotspot/os_cpu/windows_aarch64/javaThread_windows_aarch64.cpp

are a bit more extensive. That code was using an alias for `this` which is completely unnecessary (and the alias creation was using the cast). This could be factored out if thought necessary.

I grep'd as best I could for the old C-style casts but can't be certain I got them all.

Testing: 
 - all builds in our tiers1-5
 - local linux x64 product, slowdebug and fastdebug
 - GHA (TBD)
 - Sanity testing tiers 1-3
Thanks.

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Commit messages:
 - 8298241: Replace C-style casts with JavaThread::cast

Changes: https://git.openjdk.org/jdk/pull/11682/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11682&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298241
  Stats: 44 lines in 19 files changed: 0 ins; 11 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/11682.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11682/head:pull/11682

PR: https://git.openjdk.org/jdk/pull/11682