storage: fix alignement panics on 32 bit arch

[skylenet]

Attempt on fixing the following panic which is happening on the 32 bit test runs on windows:

e.g.: https://ci.appveyor.com/project/ethersphere/swarm/builds/25220636/job/ueh5r89mvapmfq48

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x0 pc=0x4028cc]
goroutine 498 [running]:
runtime/internal/atomic.Xadd64(0x13bfc62c, 0x1, 0x0, 0x4b, 0x1008)
	C:/go/src/runtime/internal/atomic/asm_386.s:105 +0xc
github.com/ethersphere/swarm/storage.(*hasherStore).storeChunk(0x13bfc600, 0xef55a0, 0x1393f240, 0xef0210, 0x1393f440)
	C:/gopath/src/github.com/ethersphere/swarm/storage/hasherstore.go:245 +0x36
github.com/ethersphere/swarm/storage.(*hasherStore).Put(0x13bfc600, 0xef55a0, 0x1393f240, 0x1c44f300, 0x4b, 0x1008, 0x13bfc6c0, 0xa, 0x10, 0x1eb, ...)
	C:/gopath/src/github.com/ethersphere/swarm/storage/hasherstore.go:84 +0x89
github.com/ethersphere/swarm/storage.(*PyramidChunker).processChunk(0x13468fc0, 0xef55a0, 0x1393f240, 0x1, 0x0, 0x1393f2c0)
	C:/gopath/src/github.com/ethersphere/swarm/storage/pyramid.go:286 +0x6d
github.com/ethersphere/swarm/storage.(*PyramidChunker).processor(0x13468fc0, 0xef55a0, 0x1393f240, 0x1, 0x0)
	C:/gopath/src/github.com/ethersphere/swarm/storage/pyramid.go:277 +0x71
created by github.com/ethersphere/swarm/storage.(*PyramidChunker).prepareChunks
	C:/gopath/src/github.com/ethersphere/swarm/storage/pyramid.go:392 +0xb9
FAIL	github.com/ethersphere/swarm	0.684s
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x0 pc=0x4028cc]

On ARM, x86-32, and 32-bit MIPS, it is the caller's responsibility to arrange for 64-bit alignment of 64-bit words accessed atomically. The first word in a variable or in an allocated struct, array, or slice can be relied upon to be 64-bit aligned.

https://golang.org/pkg/sync/atomic/#pkg-note-BUG

[skylenet]

32 bit run on Appveyor seems to have passed. I've re triggered the other jobs that failed due to a flaky test.

This also brought me to the attention on how important it is to have the CI target a 32 bit architecture so that we can identify these problems quickly.