tests: Improve backtrace-data SKIP message.

tests: Improve backtrace-data SKIP message.

The backtrace-data testcase is x86_64 linux only because it uses its
own set_initial_registers and scans its own /proc/pid/maps file.
The SKIP message it gave on other platforms was misleading. It said
"Unwinding not supported for this architecture". Change it to
"x86_64 linux only test" to be less confusing.

Signed-off-by: Mark Wielaard <mark@klomp.org>