libc/string/memset.c: Use unsigned long for stores

While most 64-bit architectures have an assembly implementation of this
file, RISC-V does not. As we now store 8 bytes instead of 4 it should speed
up RISC-V.

Reviewed By: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29536

(cherry picked from commit ab147542b7c0bbc41f7f0499b16933bd8f3f31d7)