[libc] fix -Wconversion (#77384)

Fixes the following from GCC:

    llvm-project/libc/src/string/memory_utils/op_x86.h:236:24: error:
conversion from ‘long unsigned int’ to ‘uint32_t’ {aka ‘unsigned int’}
may
    change value [-Werror=conversion]
      236 |   return (xored >> 32) | (xored & 0xFFFFFFFF);
          |          ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

Link:
https://lab.llvm.org/buildbot/#/builders/250/builds/16236/steps/8/logs/stdio
Link: https://github.com/llvm/llvm-project/pull/74506