vp8/armv6: mc: avoid boolean expression in calculation

GNU as evaluates true as '-1' while Apple's variant and llvm's internal
assembler evaluate it as '1'. The best way to avoid this madness is to
eliminate boolean expressions instead of trying to fix it with
preprocessor directives. Use a direct formula to calculate the
required temporary space on the stack in
ff_put_vp8_{epel,bilin}{4,8,16}_h[246]v[246]_armv6().

Fixes a checkasm segfault in vp8dsp.mc when using llvm's internal
assembler for a non-Apple target.
