multimedia/aom: unbreak build on riscv64
ld: error: undefined symbol: getauxval
referenced by riscv_cpudetect.c
CMakeFiles/aom_ports.dir/aom_ports/riscv_cpudetect.c.o:(riscv_simd_caps)
PR: 290172
Reported by: Rich Dunkle
(cherry picked from commit 20be7288a21a009d8bc07187a7fb87cafd04283a)