[libcxxabi][mips] Correct float_data::mangled_size for all ABI's.

Summary:
Patch by Nitesh Jain and Jaydeep Patil with a small revision to use ABIs rather
than Architecture Revisions (which currently imply particular ABIs).

Fixes test_demangle.pass.cpp (PR24149).

Subscribers: mclow.lists, jaydeep, nitesh.jain, hans, cfe-commits

Differential Revision: http://reviews.llvm.org/D11483

llvm-svn: 243645