testsuite, gdb.btrace: adjust expected output to pass with clang

testsuite, gdb.btrace: adjust expected output to pass with clang

Clang generates slightly different debug information.  Adjust the expected
output of gdb.btrace/function_call_history.exp to work with both gcc and
clang.

Also modify gdb.btrace/exception.cc to reliably trace into main and update
the corresponding patterns in gdb.btrace/exception.exp.