__builtin_readcyclecounter in include/my_rdtsc.h causes SIGILL on ARM

The commit that handles MDEV-19845 makes a mention in the commit message about how it may lead to runtime failures on ARM systems. In trying to use both the 10.4 line and 10.5 line of MariaDB on a Raspberry Pi 3 running FreeBSD, which uses an ARM Cortex-A53 processor, I am running into just that runtime failure. As it says in the subject line, I get a SIGILL when MariaDB tries to call __builtin_readcyclecounter. The easiest fix for this would be to add a preprocessor condition to the check for the builtin's availability for it to not be used when __arm__ is defined. And example of this can be found here.