x86/build: Mark per-CPU symbols as absolute explicitly for LLD

Accessing per-CPU variables is done by finding the offset of the
variable in the per-CPU block and adding it to the address of the
respective CPU's block.

Section 3.10.8 of ld.bfd's documentation states:

  For expressions involving numbers, relative addresses and absolute
  addresses, ld follows these rules to evaluate terms:

  Other binary operations, that is, between two relative addresses
  not in the same section, or between a relative address and an
  absolute address, first convert any non-absolute term to an
  absolute address before applying the operator."

Note that LLVM's linker does not adhere to the GNU ld's implementation
and as such requires implicitly-absolute terms to be explicitly marked
as absolute in the linker script. If not, it fails currently with:

  ld.lld: error: ./arch/x86/kernel/vmlinux.lds:153: at least one side of the expression must be absolute
  ld.lld: error: ./arch/x86/kernel/vmlinux.lds:154: at least one side of the expression must be absolute
  Makefile:1040: recipe for target 'vmlinux' failed

This is not a functional change for ld.bfd which converts the term to an
absolute symbol anyways as specified above.

Based on a previous submission by Tri Vo <[email protected]>.

Reported-by: Dmitry Golovin <[email protected]>
Signed-off-by: Rafael Ávila de Espíndola <[email protected]>
[ Update commit message per Boris' and Michael's suggestions. ]
Signed-off-by: Nick Desaulniers <[email protected]>
[ Massage commit message more, fix typos. ]
Signed-off-by: Borislav Petkov <[email protected]>
Tested-by: Dmitry Golovin <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Brijesh Singh <[email protected]>
Cc: Cao Jin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tri Vo <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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