[XCOFF][AIX] Use 'L..' instead of 'L' for PrivateGlobalPrefix

Without this change, names start with 'L' will get created as
temporary symbol in MCContext::createSymbol.

Some other potential prefix considered:
.L, does not work for AIX, as a function start with L will end
up with .L as prefix for its function entry point.

..L could work, but it does not play well with the convention
on AIX that anything start with '.' are considered as entry point.

L. could work, but not sure if it's safe enough, as it's possible
to have suffixes like .something append to a plain L, giving L.something
which is not necessarily a temporary.

That's why we picked L.. for now.

Differential Revision: https://reviews.llvm.org/D80831