RFR: 8320052: Zero: Use __atomic built-ins instead of __sync built-ins for better compatibility

[Aleksey Shipilev]

GCC defines `__sync*` built-ins as legacy implementations of atomic support routines, and recommends using `__atomic*` built-ins in new code: https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

The crucial difference between these two families of built-ins is that `__atomic*` 64-bit CAS/CAE is guaranteed to be supported ("The ‘__atomic’ builtins can be used with any integral scalar or pointer type that is 1, 2, 4, or 8 bytes in length"), whereas `__sync*` 64-bit CAS/CAE can be unimplemented. We are seeing Zero build failures for some platforms because of this.

This PR also cleans up the platform-specific hacks, and delegates wholesale to built-ins. I have chosen the form of `PlatformCmpxchg` -- a relaxed CAS framed by full memory barriers. This is probably overkill, but for Zero we want correctness first and foremost.

This work is important for making sure [JDK-8316961](https://bugs.openjdk.org/browse/JDK-8316961) works on all platforms, and allows [JDK-8318776](https://bugs.openjdk.org/browse/JDK-8318776) to proceed.

Additional testing:
 - [x] MacOS AArch64 Zero fastdebug; light jcstress tests still pass
 - [x] Linux x86_32 Zero release; jcstress tests pass
 - [x] Linux x86_32 Zero fastdebug, `compiler/unsafe java/lang/invoke/VarHandles`
 - [x] Linux x86_32 Zero fastdebug, bootcycle-images
 - [x] Lots of Zero builds still pass:
   * zero-fastdebug-aarch64-linux-gnu-10
   * zero-fastdebug-i686-linux-gnu-10
   * zero-fastdebug-mips64el-linux-gnuabi64-10 
   * zero-fastdebug-powerpc64le-linux-gnu-10
   * zero-fastdebug-powerpc64-linux-gnu-10
   * zero-fastdebug-riscv64-linux-gnu-10
   * zero-fastdebug-s390x-linux-gnu-10
   * zero-fastdebug-sparc64-linux-gnu-10
   * zero-fastdebug-x86_64-linux-gnu-10
 - [x] Previously broken Zero builds now pass:
   * zero-fastdebug-arm-linux-gnueabi-10
   * zero-fastdebug-arm-linux-gnueabihf-10
   * zero-fastdebug-mipsel-linux-gnu-10
   * zero-fastdebug-m68k-linux-gnu-10
   * zero-fastdebug-powerpc-linux-gnu-10
   * zero-fastdebug-sh4-linux-gnu-10

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

Commit messages:
 - Remove M68K and ARM32 cruft from BSD
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16654/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16654&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320052
  Stats: 213 lines in 4 files changed: 2 ins; 201 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/16654.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16654/head:pull/16654

PR: https://git.openjdk.org/jdk/pull/16654