gummiboot: use objcopy from the env

gummiboot: use objcopy from the env

Fix the error:
[snip]
| nm -D -u src/efi/gummiboot.so | grep ' U ' && exit 1 || :
| objcopy -j .text -j .sdata -j .data -j .dynamic \
|   -j .dynsym -j .rel -j .rela -j .reloc -j .eh_frame \
|   --target=efi-app-x86_64 src/efi/gummiboot.so gummibootx64.efi
| objcopy: src/efi/gummiboot.so: Invalid bfd target
| make[1]: *** [gummibootx64.efi] Error 1
| make[1]: *** Waiting for unfinished jobs....
| make: *** [all] Error 2
[snip]

It uses the "objcopy" directly, which is not suitable for cross compile.

[YOCTO #6054]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>