[lldb] [debugserver] Add stN aliases for stmmN for compatibility

Add stN aliases for the FPU (stmmN) registers on MacOSX.  This should
improve compatibility between MacOSX and other platforms, and partially
fix x86*-fp-write tests without having to duplicate them.  Note that
the tests are currently still broken due to ftag incompatibility.

Differential Revision: https://reviews.llvm.org/D91847