x86/asm: Use RDPKRU and WRPKRU mnemonics in <asm/special_insns.h>

Current minimum required version of binutils is 2.30, which supports RDPKRU and
WRPKRU instruction mnemonics.

Replace the byte-wise specification of RDPKRU and WRPKRU 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>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/20250616083611.157740-1-ubizjak@gmail.com