[libcxxabi] Teach cxa_demangle about Hexagon's long double size

cxa_demangle's default size for a long double is 10 bytes. Hexagon
only has an 8 byte long double though.


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@258313 91177308-0d34-0410-b5e6-96231b3b80d8