x86/idle: Use MONITORX and MWAITX mnemonics in <asm/mwait.h>

Current minimum required version of binutils is 2.30, which supports MONITORX
and MWAITX instruction mnemonics.

Replace the byte-wise specification of MONITORX and MWAITX with these proper
mnemonics.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250616083056.157460-1-ubizjak@gmail.com