botan: fix mips64 build using n32 ABI

Remove explicit -mabi parameter, and let the external toolchain wrapper set
the correct -mabi.

Fixes:
http://autobuild.buildroot.net/results/8fb/8fb8621bc5175f593de4ad0f005a5c7ffd4d9241/



Cc: Francois Perrad <fperrad@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>