[lldb] Support "eflags" register name in generic reg fallback

Enhance the generic register fallback code to support "eflags" register
name in addition to "rflags", as the former is used by gdbserver.  This
permits lldb-server to recognize the generic flags register when
interfacing with gdbserver-style target.xml (i.e. without generic=""
attributes), and therefore aligns ABI plugins' AugmentRegisterInfo()
between lldb-server and gdbserver.

Differential Revision: https://reviews.llvm.org/D108548