ABISysV_arm64: compute return value for large vectors correctly

Summary:
Arm64 Procedure Call Standard specifies than only vectors up to 16 bytes
are stored in v0 (which makes sense, as that's the size of the
register). 32-byte vector types are passed as regular structs via x8
pointer. Treat them as such.

This fixes TestReturnValue for arm64-clang. I also split the test case
into two so I can avoid the if(gcc) line, and annotate each test
instead. (It seems the vector type tests fail with gcc only when
targetting x86 arches).

Reviewers: tberghammer, eugene

Subscribers: aemerson, omjavaid, rengolin, srhines, lldb-commits

Differential Revision: https://reviews.llvm.org/D32813

llvm-svn: 302220