Adds MIPS 5.4, set default to 6.1
Adds MIPS 5.4, set default to 6.1

dkm/gcc-explorer,dkm/gcc-explorer,dkm/gcc-explorer,ibuclaw/gcc-explorer,mattgodbolt/gcc-explorer,mattgodbolt/compiler-explorer,ibuclaw/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/gcc-explorer,ibuclaw/gcc-explorer,mattgodbolt/gcc-explorer,dkm/gcc-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/gcc-explorer,dkm/gcc-explorer,ibuclaw/gcc-explorer,mattgodbolt/compiler-explorer,dkm/gcc-explorer,mattgodbolt/gcc-explorer,dkm/gcc-explorer,ibuclaw/gcc-explorer,dkm/gcc-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer,mattgodbolt/compiler-explorer