[Mips] Move MipsGOTPass implementation to the separate file.
[Mips] Rename some classes to skip redundant mentioning of 'mips'.
[Mips] Do not save GOT headers atoms into the RelocationPass class
fields.
[Mips] Rename function to better reflect its purpose.
[Mips] Do not use standard relocation identifier R_MIPS_NONE for
internal purpose. Use special LLD_R_MIPS_GLOBAL_GOT constant for that.
[Mips] Factor out the code determines type of GOT entry (local/global)
into the separate function.
[Mips] Add assert to check that we handle all paired relocations.
[Mips] GOT16 relocation against non-local symbol does not require a
paired LO16 relocation.