kbuild: Strip runtime const RELA sections correctly

Due to the fact that runtime const ELF sections are named without a
leading period or double underscore, the RSTRIP logic that removes the
static RELA sections from vmlinux fails to identify them. This results
in a situation like below, where some sections that were supposed to get
removed are left behind.

  [Nr] Name                              Type            Address          Off     Size   ES Flg Lk Inf Al

  [58] runtime_shift_d_hash_shift        PROGBITS        ffffffff83500f50 2900f50 000014 00   A  0   0  1
  [59] .relaruntime_shift_d_hash_shift   RELA            0000000000000000 55b6f00 000078 18   I 70  58  8
  [60] runtime_ptr_dentry_hashtable      PROGBITS        ffffffff83500f68 2900f68 000014 00   A  0   0  1
  [61] .relaruntime_ptr_dentry_hashtable RELA            0000000000000000 55b6f78 000078 18   I 70  60  8
  [62] runtime_ptr_USER_PTR_MAX          PROGBITS        ffffffff83500f80 2900f80 000238 00   A  0   0  1
  [63] .relaruntime_ptr_USER_PTR_MAX     RELA            0000000000000000 55b6ff0 000d50 18   I 70  62  8

So tweak the match expression to strip all sections starting with .rel.
While at it, consolidate the logic used by RISC-V, s390 and x86 into a
single shared Makefile library command.

Link: https://lore.kernel.org/all/CAHk-=wjk3ynjomNvFN8jf9A1k=qSc=JFF591W00uXj-qqNUxPQ@mail.gmail.com/


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>