syscalls.master: don't use struct siginfo

[brooks]

Summary

It doesn't exist. Instead, use struct __siginfo which does.

Rename struct siginfo32 to struct __siginfo32 to keep the ABI
transformations in makesyscalls.lua working.

[kib]

I would prefer this one split into two: siginfo32 first, the siginfo second, even if the inter-commit base cannot be built.

[brooks]

In D44380#1012169, @kib wrote:

I would prefer this one split into two: siginfo32 first, the siginfo second, even if the inter-commit base cannot be built.

I've made this split locally.