[compiler-rt] Fix compiler warnings and runtime errors in sanitizer RT strxfrm(_l) test cases.

Summary: Fixed an implicit definition warning by including <string.h>. Also fixed run-time assertions that the return value of strxfrm_l calls is less than the buffer size by increasing the size of the referenced buffer.

Reviewers: morehouse

Reviewed By: morehouse

Subscribers: dberris, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D83593