Fix raw and varargs backtrace elements

[headius]

Two fixes here:

• Fix the "raw" backtrace mode. Both "raw" and "full" were printing out all frames, but "raw" was improperly processing jitted frames into their Ruby method names like "full". I believe this was due to my unifying these two paths; "raw" passed in an empty set of interpreter frames, which prevented interpreter frames from being processed, but it still processed jitted method names. A new flag was added to indicate "raw" behavior, now skipping the jit frame rewriting.
• Fix the varargs method wrappers from JIT being included in exception traces. In Handle varargs frame on arity error in jitted code #6771 I fixed the backtrace generation to include the JIT varargs wrapper, which previously had been ignored. However I messed up the guard that prevents those frames from being processed if the wrapped method also appears in the trace (varargs frame should only show up if it appears alone, as in arity errors). The fix is to remove the expectation of the "varargs" frame marker, since they still show up as "method" type. Fixes Unexpected frame in stack trace from caller method with create_method invocation #6846.

[headius]

Second fix in action... both of these show just one "foo" frame even though they both pass through the varargs frame. The first triggers an arity error within the varargs wrapper, and the second raises within the real method body. The second case is taken from #6846.

$ jruby -e 'def foo(a); raise; end; def bar; a = [1,2]; foo(*a); end; bar'
ArgumentError: wrong number of arguments (given 2, expected 1)
     foo at -e:1
     bar at -e:1
  <main> at -e:1

$ jruby -e 'def foo(a); raise; end; def bar; a = [1]; foo(*a); end; bar'
RuntimeError: No current exception
     foo at -e:1
     bar at -e:1
  <main> at -e:1