[compiler-rt] Add internal implementations for strlcat and strlcpy

The compiler-rt should make use of strlcpy() rather than strncpy(). Using internal_strncpy() may be fine with appropriate bounds checking or enforcement of nul-termination elsewhere, but it's just good practice these days to avoid using strncpy() in new code.

A patch by Jeremy Sequoia!

Differential Revision: http://reviews.llvm.org/D14714

llvm-svn: 253690