get rid of pthread_key_(get_/set_)specific API, replace with C-style thread_local

Every major compiler has some notion of thread local storage

In C++11, there is thread_local , as per standard.

In C, in the newer standards, it is _Thread_local (C11) , or thread_local (C23) , and before that all relevant compiler have had notion of "thread local variable" for many years , i.e __thread in GCC or __declspec(thread) in MSVC,

Apart from better performance when accessing thread local variables, we'll get rid of things that depend on
initialization/cleanup of pthread_key_t variables.

"/tmp/workspace/Source-Build-Test/src/storage/perfschema/pfs.cc", line=line@entry=1379, function=function@entry=0xaaaad18df558 "void my_thread_set_THR_PFS(PFS_thread*)") at assert.c:92

#7  0x0000ffffa19e9684 in __GI___assert_fail (assertion=0xaaaad18df508 "THR_PFS_initialized", file=0xaaaad18df4c8 "/tmp/workspace/Source-Build-Test/src/storage/perfschema/pfs.cc", line=1379, function=0xaaaad18df558 "void my_thread_set_THR_PFS(PFS_thread*)") at assert.c:101