Split macro strings on whitespace

Match any whitespace instead of individual spaces. The macro
definitions in vp9/common/arm/neon/vp9_short_idct32x32_1_add_neon.asm
triggered this and treated spaces as arguments leading to lines like:
$8vld1$8.$88$8           {$8q8$8}, [$$89$8], $$8stride$8

Change-Id: I2d5718aba4614e4fd7b702e15c2a1bd80e656bd2