multimedia/libvpx: unbreak on armv7

vpx_ports/arm_cpudetect.c.o: In function `arm_cpu_caps':
arm_cpudetect.c:(.text+0x48): undefined reference to `getauxval'
arm_cpudetect.c:(.text+0x78): undefined reference to `getauxval'
c++: error: linker command failed with exit code 1 (use -v to see invocation)

PR: 224107
Reported by: pkg-fallout
Submitted by: Mikaël Urankar