Add stubs for loong64 arithmetic routines

[abner-chenc]

loong64 GOARCH value reserved for LoongArch architecture: https://golang.org/doc/go1.17.

github issues: https://github.com/golang/go/issues/46229

golang source code: https://github.com/loongson/go/tree/loong64-master

LoongArch documents: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html

go test: === RUN TestCalibrateThreshold --- PASS: TestCalibrateThreshold (0.00s) === RUN TestCalibrateFFT --- PASS: TestCalibrateFFT (0.00s) === RUN TestFermatShift --- PASS: TestFermatShift (0.01s) === RUN TestFermatShiftHalf --- PASS: TestFermatShiftHalf (0.22s) === RUN TestFermatAdd --- PASS: TestFermatAdd (0.00s) === RUN TestFermatMul --- PASS: TestFermatMul (0.00s) === RUN TestRoundTripIntPoly --- PASS: TestRoundTripIntPoly (0.88s) === RUN TestFourierSizes --- PASS: TestFourierSizes (0.26s) === RUN TestFourier --- PASS: TestFourier (0.32s) === RUN TestRoundTripPolyValues --- PASS: TestRoundTripPolyValues (0.04s) === RUN TestMul --- PASS: TestMul (3.12s) === RUN TestIssue1 --- PASS: TestIssue1 (0.01s) === RUN TestScan --- PASS: TestScan (14.74s) PASS ok github.com/remyoudompheng/bigfft 19.607s

[mei-rune]

can merge it?