x86: Use POPCNT mnemonics in arch_hweight.h

Recently, the minimum required version of binutils was changed to
2.20, which supports POPCNT instruction mnemonics.

Replace the byte-wise specification of POPCNT with those proper
mnemonics.

 [ bp: massage commit message and remove line breaks. ]

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Ingo Molnar <mingo@redhat.com>
CC: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20181014202354.21281-1-ubizjak@gmail.com