klibc: define KLIBCLIBGCC

Fix build of klibc extending the EXTRA_OEMAKE.

|   LIST    usr/klibc/klib.list
|   KLIBCAR usr/klibc/libc.a
|   KLIBCLD usr/klibc/libc.so
| arm-oe-linux-gnueabi-ld: cannot find libgcc.a: No such file or directory
| make[2]: *** [usr/klibc/libc.so] Error 1
| make[2]: *** Waiting for unfinished jobs....
| make[1]: *** [all] Error 2
| make: *** [klibc] Error 2
| ERROR: oe_runmake failed

Signed-off-by: Andrea Adami <andrea.adami@gmail.com>