Remove unnecessary mbstring skipifs

These functions are always available (if the extension is available
at all).

Migrate more SKIPIF -> EXTENSIONS (#7139)

This is a mix of more automated and manual migration. It should remove all applicable extension_loaded() checks outside of skipif.inc files.

Allow empty needle in mb_strrchr()

Allow empty needles in mb_strpos and mb_strstr function family.

MBstring analogous implementation to 6d578482a933be7597b686b59a935b316161d251

Closes GH-4977