Make MAX_CONTEXT_SIZE dynamically sized to future-proof it

On Skylake this assert fires:

Internal Error: DynamoRIO debug check failure: D:\dynamorio_package\core\win32\ntdll.c:5359 res >= 0 && len <= MAX_CONTEXT_SIZE

Xref #1937 (closed) for the corresponding Linux signal context issue. We need to make MAX_CONTEXT_SIZE be dynamically sized.