pixman: Enable '--disable-arm-iwmmxt' on ARM

Otherwise the build fails on ARMv6:

  CC       libpixman_iwmmxt_la-pixman-mmx.lo
pixman-mmx.c: In function 'mmx_fetch_x8r8g8b8':
pixman-mmx.c:3812:1: internal compiler error: Max. number of generated reload insns per insn is achieved (90)