multimedia/libvpx: unbreak armv6/armv7 on -CURRENT

as -I./ -I"/wrkdirs/usr/ports/multimedia/libvpx/work/libvpx-1.9.0"/ -march=armv7-a -meabi=5 -mfpu=neon -o vpx_dsp/arm/vpx_convolve_copy_neon_asm.asm.S.o vpx_dsp/arm/vpx_convolve_copy_neon_asm.asm.S
/bin/sh: as: not found

PR: 250045
Submitted by: mikael (based on)