[Mips] Make mips32/mips32r2/mips64/mips64r2 real aliases for appropriate
march options.
[Mips] Do not check for options from the OPT_mips_CPUs_Group. All these
options are aliases now.
[Mips] Remove unused mips_CPUs_Group options group. It's better to join
all MIPS-related options to the new m_mips_Features_Group later.
[Mips] Make the isMipsR2Arch() function simpler - remove checking for
MIPS-arch.
[Mips] Group MIPS-related options checking function in one place.