StdLib: Fix pointer arithmetic issues in the strncasecmp function.

StdLib: Fix pointer arithmetic issues in the strncasecmp function.

The original Linux code tried to be too fancy so the internal pointers got out of sync.
Rewrote the function to at least be more clear.
Regardless, it now works properly.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Daryl McDaniel <daryl.mcdaniel@intel.com>
Reviewed by: matthew.stanbro@intel.com
Reviewed by: erik.c.bjorge@intel.com

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14664 6f19259b-4bc3-4df7-8a09-765794883524