LibC: Don't delete null check in `gettimeofday`

The `nonnull` attribute may delete null checks in the generated code, as
per the [GCC documentation]:

> The compiler may also perform optimizations based on the knowledge
> that nonnul parameters cannot be null. This can currently not be
> disabled other than by removing the nonnull attribute.

Disassembling the function as compiled by GCC, we can see that there is
no branch based on if `tv` is null.  This means that `gettimeofday`
would produce UB if passed a null parameter, even if we wanted to
predictably return an error.  Clang refuses to compile this due to a
`pointer-bool-conversion` warning.

In this commit, `settimeofday` is changed as well to match
`gettimeofday`'s null argument handling.

[GCC documentation]:
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-nonnull-function-attribute

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