Split mangle test into two files

We currently have two VarDeclarations in mangle.d which reference the same symbol. This causes problems in the GCC backend as it does not realize it actually deals with the same symbol and therefore
reorders reads and writes. It's not clear if this will be fixed in the GCC backend at all as the fix would break other stuff. It's also kind of weird to reference a symbol in this way. The correct way is to use aliases.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60214

Anyway, this test was likely not supposed to test if the compiler recognizes that these are the same variables, it was probably just to test that we can read/write to a symbol with a specific mangle.

@IgorStepanov can you comment on the intention of the test?