build: Fix ARM ISA detection

armv7 isn't a real cpu_family in meson, so drop that. The detection for
__ARM_ARCH_ISA_ARM was also inverted.

Fixes: pulseaudio/webrtc-audio-processing#6
4 jobs for fix-arm-build in 4 minutes and 15 seconds
Status Job ID Name Coverage
  Container
passed #6125489
aarch64
build-container-aarch64

00:00:20

passed #6125488
build-container-x86_64

00:00:17

 
  Build
passed #6125491
aarch64
build-aarch64

00:00:51

passed #6125490
build-x86_64

00:03:53