Linker cache should not take layout names into account

Layout names are of no consequence to the linker, however, the current DOWNCALL_CACHE and UPCALL_CACHE in AbstractLinker inadvertantly factor these layout names into the keys of the cache, as the cache key type, LinkRequest, uses FunctionDescriptor's hashcode/equals, which uses MemoryLayout's hashcode/equals, which relies on layout names.

LinkeRequest should override hashcode and equals to ignore layout names.