arm64: change configure dotprod test to a run test

arm64: change configure dotprod test to a run test

We were testing for dotprod with AC_COMPILE_IFELSE. That's wrong, it just
checks that the compiler accepts -march=armv8.2-a+dotprod and that the
assembler handles dotprod opcodes. Changed it to AC_RUN_IFELSE which
additionally checks that the binary runs. Valgrind should be checking
for this in VG_(machine_get_hwcaps), storing it in VexArchInfo and
checking it in guest_arm64_toIR.c (it looks like we check it but don't
store 'dp' in vai.hwcaps and in general VEX doesnt check whether it
should try to decode these extensions).