x86/asm: Remove code depending on __GCC_ASM_FLAG_OUTPUTS__

The minimum supported GCC version is 8.1, which supports flag output operands
and always defines __GCC_ASM_FLAG_OUTPUTS__ macro.

Remove code depending on __GCC_ASM_FLAG_OUTPUTS__ and use the "=@ccCOND" flag
output operand directly.

Use the equivalent "=@ccz" instead of "=@cce" flag output operand for
CMPXCHG8B and CMPXCHG16B instructions. These instructions set a single flag
bit - the Zero flag - and "=@ccz" is used to distinguish the CC user from
comparison instructions, where set ZERO flag indeed means that the values are
equal.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250905121723.GCaLrU04lP2A50PT-B@fat_crate.local