Breakdown metrics causes panic on 32-bit arch

[axw]

Describe the bug

Breakdown metrics are causing segfaults on 32-bit systems.

To Reproduce

Run the following program on a 32-bit system, or in 32-bit mode (e.g. GOARCH=386 go run ...):

package main

import "go.elastic.co/apm/apmtest"

func main() {
        t := apmtest.DiscardTracer
        for {
                tx := t.StartTransaction("name", "type")
                span := tx.StartSpan("name", "type", nil)
                span.End()
                tx.End()
        }
}

The program will quickly segfault, due to misalignment:

anic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x8049fec]

goroutine 34 [running]:
runtime/internal/atomic.Xadd64(0x93f805c, 0x0, 0x0, 0xffffffff, 0x0)
        /home/andrew/tools/go/1.13.0/src/runtime/internal/atomic/asm_386.s:105 +0xc
go.elastic.co/apm.(*breakdownTiming).accumulate(0x93f805c, 0x0, 0x0, 0x0, 0x0, 0x336, 0x0, 0x1)
        /home/andrew/go/src/go.elastic.co/apm/breakdown.go:167 +0x4e
go.elastic.co/apm.(*breakdownMetricsMap).record(0x90b08d0, 0x836ed0d, 0x4, 0x836ec59, 0x4, 0x836ed0d, 0x4, 0x0, 0x0, 0x0, ...)
        /home/andrew/go/src/go.elastic.co/apm/breakdown.go:230 +0x1b1
go.elastic.co/apm.(*breakdownMetrics).recordTransaction(0x90b0930, 0x9082a80, 0x0)
        /home/andrew/go/src/go.elastic.co/apm/breakdown.go:211 +0x29a
go.elastic.co/apm.(*Tracer).loop(0x914a0f0)
        /home/andrew/go/src/go.elastic.co/apm/tracer.go:841 +0x1c56
created by go.elastic.co/apm.newTracer
        /home/andrew/go/src/go.elastic.co/apm/tracer.go:410 +0x615
exit status 2

Setting ELASTIC_APM_BREAKDOWN_METRICS=false in the environment stops this from occurring.

Expected behavior

The program should not crash.