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.
4 jobs for fix-arm-build in 1 minute and 41 seconds (queued for 1 second)
Status Name Job ID Coverage
  Container
passed build-container-aarch64 #6125485
aarch64

00:00:37

passed build-container-x86_64 #6125484

00:00:46

 
  Build
passed build-aarch64 #6125487
aarch64

00:00:50

passed build-x86_64 #6125486

00:00:54