Fix ep_rt_atomic_compare_exchange_size_t in Mono EventPipe shim. (#58369)

Fix ep_rt_atomic_compare_exchange_size_t in Mono EventPipe shim. (#58369)

ep_rt_atomic_compare_exchange_size_t was always truncated to gint32 on
Mono 64-bit platforms. This commit use 32 or 64-bit cas version
depending on SIZEOF_SIZE_T. NOTE, CoreCLR used a template function for
this and already picks 32 or 64 bit version depending on size of size_t.

ep_rt_atomic_compare_exchange_size_t is currently used to reflect size
of EventPipe buffer manager buffers and will most likely never need
more than 2GB of data, but better to fix it and inline with CoreCLR shim
implementation.

Co-authored-by: lateralusX <lateralusx.github@gmail.com>