[dev.regabi] cmd/internal/obj: access Attribute atomically
[dev.regabi] cmd/internal/obj: access Attribute atomically

Symbol's Attributes and ABI are in the same word. In the
concurrent backend, we may read one symbol's ABI (the callee)
while setting its attributes in another goroutine.

Fix racecompile build.

Change-Id: I500e869bafdd72080119ab243db94eee3afcf926
Reviewed-on: https://go-review.googlesource.com/c/go/+/289290
Trust: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

nawawi/go,golang/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,golang/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go