Added generic register numbers for simple ABI argument registers and defined

the appropriate registers for arm and x86_64. The register names for the
arguments that are the size of a pointer or less are all named "arg1", "arg2",
etc. This allows you to read these registers by name:

(lldb) register read arg1 arg2 arg3
...

You can also now specify you want to see alternate register names when executing
the read register command:

(lldb) register read --alternate
(lldb) register read -A

llvm-svn: 131376