riscv: Add static asssert for context size

[imp]

Summary

Add a static assert for the mcontext_t and ucontext_t sizes. These are
de-factor ABI options and cannot change size ever.

Sponsored by: Netflix

[imp]

update, per review

[imp]

update, per review (for real this time)

[emaste]

IMO "has incorrect size" would be more clear than "bogusly changed size"

[imp]

In D34214#774178, @emaste wrote:

IMO "has incorrect size" would be more clear than "bogusly changed size"

I'll make this change globally across all the reviews, but I won't upload them again unless other changes are needed.
Reviewers should assume that change is made.

[emaste]

I'll make this change globally across all the reviews, but I won't upload them again unless other changes are needed.

👍

[andrew]

sys/riscv/riscv/exec_machdep.c

81

Shouldn't this message be ucontext_t ...?

[imp]

update per review