polarssl: disable assembly for MIPS R6

Fixes:
http://autobuild.buildroot.net/results/c34/c343c68d3e0dae4a7ecd59693298a9622bc56662/



Is not yet supported and the compilation will fail like this:

[  4%] Building C object library/CMakeFiles/polarssl.dir/bignum.c.o
/tmp/ccLDxl9G.s: Assembler messages:
/tmp/ccLDxl9G.s:92: Error: opcode not supported on this processor:
mips32r6 (mips32r6) `multu $13,$14'
/tmp/ccLDxl9G.s:93: Error: opcode not supported on this processor:
mips32r6 (mips32r6) `addi $10,$10,4'
/tmp/ccLDxl9G.s:94: Error: opcode not supported on this processor:
mips32r6 (mips32r6) `mflo $14'
/tmp/ccLDxl9G.s:95: Error: opcode not supported on this processor:
mips32r6 (mips32r6) `mfhi $9'

[...]

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>