glibc: Pass correct -mabi for MIPS

According to glibc-2.18/ports/sysdeps/mips/preconfigure,
if no -mabi was passed to CFLAGS, then it defaults to -mabi=n32.
This breaks o32 and n64 builds for MIPS64. Therefore, it is
necessary to append -mabi to CFLAGS.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>