objtool: Add action to check for absence of absolute relocations

The x86 startup code must not use absolute references to code or data,
as it executes before the kernel virtual mapping is up.

Add an action to objtool to check all allocatable sections (with the
exception of __patchable_function_entries, which uses absolute
references for nebulous reasons) and raise an error if any absolute
references are found.

Note that debug sections typically contain lots of absolute references
too, but those are not allocatable so they will be ignored.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/20250828102202.1849035-39-ardb+git@google.com