Add NaCl (NativeClient) specific classes Target_mips_nacl and

Add NaCl (NativeClient) specific classes Target_mips_nacl and
Target_selector_mips_nacl.

gold/
	* mips.cc (Target_mips_nacl): New class.
	(Target_selector_mips_nacl): New class.
	(target_selector_mips32): Rename from target_selector_mips32be and use
	Target_selector_mips_nacl instead of Target_selector_mips.
	(target_selector_mips32el): Rename from target_selector_mips32 and use
	Target_selector_mips_nacl instead of Target_selector_mips.
	(target_selector_mips64): Rename from target_selector_mips64be and use
	Target_selector_mips_nacl instead of Target_selector_mips.
	(target_selector_mips64el): Rename from target_selector_mips64 and use
	Target_selector_mips_nacl instead of Target_selector_mips.
	(Target_mips::mips_info): Add const attribute.