x86: Fix car_uninit weak symbol definition

Since commit 80df194f ("x86: detect unsupported relocation types"),
an error message is seen on QEMU x86 target during boot:

do_elf_reloc_fixups32: unsupported relocation type 0x1 at fff841f0, offset = 0xfff00087
do_elf_reloc_fixups32: unsupported relocation type 0x2 at fff841f8, offset = 0xfff00091

Check offset 0xfff00087 and 0xfff00091 in the u-boot ELF image,

fff00087  000df401 R_386_32          00000000   car_uninit
fff00091  000df402 R_386_PC32        00000000   car_uninit

we see R_386_32 and R_386_PC32 relocation type is generated for
symbol car_uninit, which is declared as a weak symbol in start.S.

However the actual weak symbol implementation ends up nowhere. As
we can see below, it's *UND*.

$ objdump -t u-boot | grep car_uninit
00000000  w      *UND*  00000000 car_uninit

With this fix, it is normal now.

$ objdump -t u-boot | grep car_uninit
fff00094  w    F .text.start    00000001 car_uninit

Reported-by: Hannes Schmelzer <hannes@schmelzer.or.at>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Hannes Schmelzer <oe5hpm@oevsv.at>