base64x use avx2 simd cause panic

[liuq19]

use avx2 mode in base64

const (
    _MODE_JSON = (1 << 3) | (1 << 2) // base64 mode
)

test command:

go test -run=TestIssue213$  -count 100 -v  > debug.out 

output:

Begin GC looping...
=== RUN   TestIssue213
--- PASS: TestIssue213 (0.03s)
=== RUN   TestIssue213
--- PASS: TestIssue213 (0.00s)
=== RUN   TestIssue213
fatal error: unexpected signal during runtime execution
[signal SIGSEGV: segmentation violation code=0x1 addr=0xb01dfacedebac1e pc=0x102410b]

runtime stack:
runtime.throw({0x13a029a, 0x80c004779fff})
	/Users/bytedance/goroot/go1.17/src/runtime/panic.go:1198 +0x71
runtime.sigpanic()
	/Users/bytedance/goroot/go1.17/src/runtime/signal_unix.go:719 +0x396
runtime.(*sweepLocked).sweep(0x293305b8, 0x0)
	/Users/bytedance/goroot/go1.17/src/runtime/mgcsweep.go:436 +0x12b
runtime.(*mcentral).uncacheSpan(0x104e5c6, 0x70000c064e80)
	/Users/bytedance/goroot/go1.17/src/runtime/mcentral.go:223 +0