x86/percpu: Use the correct asm operand modifier in percpu_stable_op()

The "P" asm operand modifier is a x86 target-specific modifier.

When used for a constant, it drops all syntax-specific prefixes and
issues the bare constant. This modifier is not correct for address
handling, in this case a generic "a" operand modifier should be used.

The "a" asm operand modifier substitutes a memory reference, with the
actual operand treated as address.  For x86_64, when a symbol is
provided, the "a" modifier emits "sym(%rip)" instead of "sym",
enabling shorter %rip-relative addressing.

Clang allows only "i" and "r" operand constraints with an "a" modifier,
so the patch normalizes the modifier/constraint pair to "a"/"i"
which is consistent between both compilers.

The patch reduces code size of a test build by 4072 bytes:

   text            data     bss    dec             hex     filename
   25523268        4388300  808452 30720020        1d4c014 vmlinux-old.o
   25519196        4388300  808452 30715948        1d4b02c vmlinux-new.o

[ mingo: Changelog clarity. ]
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20231016200755.287403-1-ubizjak@gmail.com

arch/x86/include/asm/percpu.h

@@ -194,9 +194,9 @@ do {									\
 #define percpu_stable_op(size, op, _var)				\
 ({									\
 	__pcpu_type_##size pfo_val__;					\
-	asm(__pcpu_op2_##size(op, __force_percpu_arg(P[var]), "%[val]")	\
+	asm(__pcpu_op2_##size(op, __force_percpu_arg(a[var]), "%[val]")	\
 	    : [val] __pcpu_reg_##size("=", pfo_val__)			\
-	    : [var] "p" (&(_var)));					\
+	    : [var] "i" (&(_var)));					\
 	(typeof(_var))(unsigned long) pfo_val__;			\
 })