Disable PIE for MIPS BERI boot loader

MIPS BERI boot loader is built with -mno-abicalls and -fno-pic
so prevent adding PIE-related build flags.

Fix build after 9a227a2fd642 ("Enable PIE by default on 64-bit architectures")

Obtained from: Semihalf
Sponsored by: Stormshield

(cherry picked from commit 19cd12a32afe801dc5e753013993e15e924ceb39)