grub-lakitu: FROMGIT: x86-64: Treat R_X86_64_PLT32 as R_X86_64_PC32

Starting from binutils commit bd7ab16b4537788ad53521c45469a1bdae84ad4a:

https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=bd7ab16b4537788ad53521c45469a1bdae84ad4a

x86-64 assembler generates R_X86_64_PLT32, instead of R_X86_64_PC32, for
32-bit PC-relative branches.  Grub2 should treat R_X86_64_PLT32 as
R_X86_64_PC32.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
(cherry picked from commit 842c390469e2c2e10b5aa36700324cd3bde25875
 https://git.savannah.gnu.org/git/grub.git master)
Signed-off-by: Robert Kolchmeyer <rkolchmeyer@google.com>

BUG=b/162782466
TEST=shielded_vm_test

Change-Id: Idb3bbdf6be15f2f0317b42a6e22e79de66f6103e