closed #4383: Fixed missing letters if using old LabelTTF and running on
iphone 64bit simulator(device).
fonts/xxx.ttf -> Xxx for old LabelTTF test.
Label:fixed possible appear OpenGL error 0x500.
Renames s_isIOS7 to s_isIOS7OrHigher
remove unused local variable.
Merge pull request #6090 from dumganhar/iss4383-label-missing-letters

closed #4383: Fixed missing letters if using old LabelTTF and running on  iPhone 64bit simulator(device).
Merge pull request #6091 from Dhilan007/develop_label

Label:fixed possible appear OpenGL error 0x500.
Update CHANGELOG [ci skip]