libc/glibc: set the endian option according to extra CFLAGS

When building a multilib, some extra CFLAGS can override the
default config option. This is the case for the endianness
selection.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Thomas Petazzoni"

 reporter_url="http://www.free-electrons.com/"

 reporter_comment="Very basic testing only"