Use TargetData to obtain the correct size of the "size_t" argument for

Use TargetData to obtain the correct size of the "size_t" argument for
functions like memcpy, memmove and memset. Ensure only one prototype is
used for these functions and that it matches the system definition by
using the appropriate type for the size argument.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33618 91177308-0d34-0410-b5e6-96231b3b80d8