fix build with gccgo

[kmlebedev]

# github.com/remyoudompheng/bigfft
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s: Assembler messages:
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:8: Error: no such instruction: `text ·addVV(SB),4,$0'
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:9: Error: junk `(SB)' after expression
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:13: Error: no such instruction: `text ·subVV(SB),4,$0'
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:14: Error: junk `(SB)' after expression
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:17: Error: no such instruction: `text ·addVW(SB),4,$0'
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:18: Error: junk `(SB)' after expression
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:22: Error: no such instruction: `text ·subVW(SB),4,$0'
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:23: Error: junk `(SB)' after expression
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:26: Error: no such instruction: `text ·shlVU(SB),4,$0'
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:27: Error: junk `(SB)' after expression
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:30: Error: no such instruction: `text ·shrVU(SB),4,$0'
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:31: Error: junk `(SB)' after expression
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:34: Error: no such instruction: `text ·mulAddVWW(SB),4,$0'
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:35: Error: junk `(SB)' after expression
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:38: Error: no such instruction: `text ·addMulVVW(SB),4,$0'
/root/go/pkg/mod/github.com/remyoudompheng/bigfft@v0.0.0-20200410134404-eec4a21b6bb0/arith_amd64.s:39: Error: junk `(SB)' after expression

[kmlebedev]

@remyoudompheng Hi, your project has a lot of dependencies, please consider PR