XEmitter: Add enum class Jump

Replace the bool parameter force5bytes in J, JMP, and J_CC with an enum
class Jump::Short/Near. Many callers set that parameter to the literal
'true', which was unclear if you didn't already know what it did.

Source/UnitTests/Common/x64EmitterTest.cpp

@@ -292,7 +292,7 @@ TEST_F(x64EmitterTest, JMP)
                     "jmp .-8");
 
   emitter->NOP(6);
-  emitter->JMP(code_buffer, true);
+  emitter->JMP(code_buffer, XEmitter::Jump::Near);
   ExpectDisassembly("multibyte nop "
                     "jmp .-11");
 }