Rename lldb registers to contain lldb_ prefix.

LLDB supports many different register numbering schemes, and these
are typically prefixed with an indicator that lets the user know
what numbering scheme is used.  The gcc numbering scheme is
prefixed with gcc, and there are similar ones for dwarf, gdb,
and gcc_dwarf.

LLDB also contains its own internal numbering scheme, but the enum
for LLDB's numbering scheme was prefixed differently.  This patch
changes the names of these enums to use the same naming scheme for
the enum values as the rest of the register kinds by removing gpr_
and fpu_ prefixes, and instead using lldb_ prefixes for all enum
values.

Differential Revision: http://reviews.llvm.org/D6351
Reviewed by: Greg Clayton