RFR: JDK-8309462: vmTestbase/nsk/jvmti/RunAgentThread/agentthr001/TestDescription.java crashing due to empty while loop

[JoKern65]

The sys_thread_3() function contains an empty while loop, which by the standard can be optimized away. Please refer to discussion in https://github.com/llvm/llvm-project/issues/60622
The xlc17 compiler is doing so, and IBM claims that they are following the standard and will not fix this on compiler side.
So we have (at least) 3 ways to circumvent this behavior.

1. we can introduce the call of a nop library function, which will hinder the optimizer to throw away the loop (This is our proposed solution, but instead of a heavy looping thread, the result is a more or less idle thread):
`#include <unistd.h>`
`static void`
`sys_thread_3(jvmtiEnv* jvmti, JNIEnv* jni, void *p)`
`{`
`                while (1) {`
`                  sleep(1);`
`                }`
`}`

2. We can make use of a volatile variable in the loop body which also hinders the optimizer to throw away the loop:
`static void`
`sys_thread_3(jvmtiEnv* jvmti, JNIEnv* jni, void *p)`
`{`
`                volatile int i = 1;`
`                while (i) {`
`                  i += 2;`
`                }`
`}`

3. we can use the __attribute__ ((optnone)) modifier in the function declaration to suppress the optimization at all:
`static void`
`sys_thread_3(jvmtiEnv* jvmti, JNIEnv* jni, void *p) __attribute__ ((optnone))`
`{`
`                while (1) {`
`                }`
`}`

To make the third approach platform independent, we can implement it in the following way:
In globalDefinitions.hpp
`#ifndef OPTNONE`
`#define OPTNONE`
`#endif`

In globalDefinitions_xlc.hpp
`// optnone support`
`//`
`// To use if a function should not be optimized`
`// Usage:`
`// void* func(size_t size) OPTNONE {...}`
`#define OPTNONE __attribute__(( optnone))``

With this we can change libagentthr001.cpp in a platform independent way to
`static void`
`sys_thread_3(jvmtiEnv* jvmti, JNIEnv* jni, void *p) OPTNONE`
`{`
`     while (1) {`
`     }`
`}`

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

Commit messages:
 - JDK-8309462

Changes: https://git.openjdk.org/jdk/pull/14330/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14330&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309462
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14330.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14330/head:pull/14330

PR: https://git.openjdk.org/jdk/pull/14330