[LibOS] regression: fix GDB tests

GDB tests expect `_start()` function in the backtrace. However, it
depends on the GDB version and on the setting `backtrace past-main
off|on` (which seems to differ in different GDB versions). So this
commit explicitly enables past-main traces.

Signed-off-by: Dmitrii Kuvaiskii <dmitrii.kuvaiskii@intel.com>

libos/test/regression/debug.gdb

@@ -1,5 +1,6 @@
 set breakpoint pending on
 set pagination off
+set backtrace past-main on
 
 # Check if debug sources are loaded in our program, and we can break inside.