x86-64: Replace movzx with movzbl

Clang cannot assemble movzx in the AT&T dialect mode.

../sysdeps/x86_64/strcmp.S:2232:16: error: invalid operand for instruction
 movzx (%rsi), %ecx
               ^~~~

Change movzx to movzbl, which follows the AT&T dialect and is used
elsewhere in the file.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>