MFH: r480912

gecko: whitelist where DTRACE is enabled by default

Userland probes are only supported on Tier1 architectures. Even though
powerpc* should support dtrace -G may crash on C++ files built by GCC.
DTRACE option is still exposed as long as dtrace(1) binary is present
for easier debugging of libdtrace.

PR: 221641 224271
Approved by: ports-secteam blanket