[lldb] [Process/NetBSD] Fix wrongly mapping mm* registers

Fix mistake that mapped mm* registers into the space for xmm* registers,
rather than the one shared with st* registers.  In other words,
'register read mmN' now correctly shows the mmN register rather than
part of xmmN.

Includes a minimal lit regression test.

Differential Revision: https://reviews.llvm.org/D60325

llvm-svn: 358178