fix atomic alignment on 32-bit platforms

[yplam]

On ARM, 386, 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.

[Danielius1922]

@jkralik Hmm, UBSan in C++ does alignment checks, maybe there's some Go tool that does it as well? Or at least we should probably run tests on a 32bit platform.

[ihidchaos]

Hi @jkralik @Danielius1922 developers， I also met this error on my OpenWRT platform.

[root@GW:/tmp/tmp]#cat /proc/cpuinfo 
system type		: MediaTek MT7628AN ver:1 eco:2
machine			: Jotale JS76x8 (32M)
processor		: 0
cpu model		: MIPS 24KEc V5.5
BogoMIPS		: 382.46
wait instruction	: yes
microsecond timers	: yes
tlb_entries		: 32
extra interrupt vector	: yes
hardware watchpoint	: yes, count: 4, address/irw mask: [0x0ffc, 0x0ffc, 0x0ffb, 0x0ffb]
isa			: mips1 mips2 mips32r1 mips32r2
ASEs implemented	: mips16 dsp
Options implemented	: tlb 4kex 4k_cache prefetch mcheck ejtag llsc pindexed_dcache userlocal vint perf_cntr_intr_bit perf
shadow register sets	: 1
kscratch registers	: 0
package			: 0
core			: 0
VCED exceptions		: not available
VCEI exceptions		: not available

goroutine 13 [running]:
runtime/internal/atomic.panicUnaligned(...)
	C:/Program Files/Go/src/runtime/internal/atomic/unaligned.go:8
runtime/internal/atomic.lockAndCheck(...)
	C:/Program Files/Go/src/runtime/internal/atomic/atomic_mipsx.go:38
runtime/internal/atomic.Load64(0x2a2fd54)
	C:/Program Files/Go/src/runtime/internal/atomic/atomic_mipsx.go:94 +0xa4
github.com/plgd-dev/go-coap/v2/udp/message/pool.(*Pool).ReleaseMessage(0x2a2fd40, 0x2a6e420)
	C:/Users/IHID/go/pkg/mod/github.com/plgd-dev/go-coap/v2@v2.5.1-0.20220331091656-0f32afde2208/udp/message/pool/message.go:197 +0x48
github.com/plgd-dev/go-coap/v2/udp/client.(*ClientConn).ReleaseMessage(...)
	C:/Users/IHID/go/pkg/mod/github.com/plgd-dev/go-coap/v2@v2.5.1-0.20220331091656-0f32afde2208/udp/client/clientconn.go:786
github.com/plgd-dev/go-coap/v2/udp/client.(*ClientConn).Process.func2.2(0x2874eb0, 0x2a58ce0)
	C:/Users/IHID/go/pkg/mod/github.com/plgd-dev/go-coap/v2@v2.5.1-0.20220331091656-0f32afde2208/udp/client/clientconn.go:734 +0x4c
github.com/plgd-dev/go-coap/v2/udp/client.(*ClientConn).Process.func2()
	C:/Users/IHID/go/pkg/mod/github.com/plgd-dev/go-coap/v2@v2.5.1-0.20220331091656-0f32afde2208/udp/client/clientconn.go:744 +0x458
github.com/plgd-dev/go-coap/v2/udp.glob..func2.2.1(0x2a58cd0)
	C:/Users/IHID/go/pkg/mod/github.com/plgd-dev/go-coap/v2@v2.5.1-0.20220331091656-0f32afde2208/udp/server.go:54 +0x3c
created by github.com/plgd-dev/go-coap/v2/udp.glob..func2.2
	C:/Users/IHID/go/pkg/mod/github.com/plgd-dev/go-coap/v2@v2.5.1-0.20220331091656-0f32afde2208/udp/server.go:53 +0x50

I have tested that this pull can solve it, can you merge it to the master branch?
Best Regards.
Guo

[jkralik]

Thx for contributing.