cc: Work around verifier error when reading USDT probe arguments

cc: Work around verifier error when reading USDT probe arguments

The code generated by the USDT helpers for reading probe arguments
occasionally caused a verifier error due to a construct similar to
the following:

```
switch (ctx->ip) {
case 0xaaaa: *dest = ctx->cx; break;
case 0xbbbb: *dest = ctx->dx; break;
}
```

This would generate an instruction sequence that attempts to access
ctx through an offset that is not statically known, which confuses
the verifier. This was reported in #751, #829, and #1133, and likely
seen by others as well.

The workaround, suggested by @yonghong-song, is to force memory
writes by using the volatile modifier, which precludes this specific
optimization.

Resolves #751, #829, #1133.