x86/runtime-const: Add the RUNTIME_CONST_PTR assembly macro

Add an assembly macro to refer runtime cost. It hides linker magic and
makes assembly more readable.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250304153342.2016569-1-kirill.shutemov@linux.intel.com