mpir: fix build on mips32r6/mips64r6

mpir: fix build on mips32r6/mips64r6

The optimized mips32 and mips64 assembly code is not compatible with
mips32r6/mips64r6, so use the generic code of MPIR on such platforms.

Fixes:

 - http://autobuild.buildroot.net/results/db2542a2ec3029eda81738a7ae5faeb9ea4aedee/
   mips32r6 toolchain

 - http://autobuild.buildroot.net/results/50b94e6d0dbc33664c6bfe06f83b211083aa07f7/
   mips64r6 toolchain

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>