Validate that rules without toolchain context cannot create actions

[katre]

In theory, RuleContext.getToolchainContext will only return null if the rule is not generating actions (as set in RuleClass.Builder.useToolchainResolution(ToolchainResolutionMode.DISABLED)). However, we aren't currently validating this, and it's possible that legacy rules are violating this.

We should add explicit validation around this to ensure that any rule which generates actions has a toolchain context (which includes both the target and exec platforms for that configured target).

[katre]

Okay, did some testing on #20525 (downstream tests) and the google-internal version, and the only issue is that ~every rule creates file write actions, even if that rule doesn't use toolchains (and thus has no execution platform).

Really, file writes should have the host platform as the exec platform anyway, so this is a) ignorable, and b) trivially fixable.

However, there's no good way to add ongoing monitoring short of crashing the Bazel server, because RuleContext.registerAction is incredibly deeply used: after adding throws RuleErrorException to registerAction I had to update over 140 other files (to declare that it could be thrown, or to handle it), and I was still getting compilation errors when I gave up.