[TLI] Add memset_pattern4, memset_pattern8 lib functions.

Similar to memset_pattern16, memset_pattern4, memset_pattern8 are
available on Darwin platforms.

https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/memset_pattern4.3.html

Reviewed By: ab

Differential Revision: https://reviews.llvm.org/D114881