Add Structures for lock-free initialization

[josephlr]

Fixes #50

This is the first step to improving the synchronization concerns in getrandom. Currently we need synchronization to:

1. Hold a global file descriptor for /dev/urandom or /dev/random
2. "Weak linkage" of a libc symbol (getrandom on solaris or getentropy on macos).
3. Detecting RDRAND
4. Detecting getrandom(2) on Linux.
5. Determining the environment on wasm32-unknown-unknown.

Right now we use lazy_static for all of these, but that isn't ideal if we want getrandom to be used to implement libstd. To address some of these concerns, we add a LazyUsize type that just wraps an AtomicUsize, and provides methods for lazy initialization, where it is OK for the initialization to be run multiple times. We then build two structs on top of LazyUsize:

1. LazyBool - This is just like LazyUsize except with bool types in the function signatures. It is used to replace lazy_static for (3) and (4).
2. Weak - Based on the libstd struct of the same name, this allows us to "link" to system libc functions that may or may not be present at runtime. This replaces lazy_static for (2). It also simplifies some duplicated logic, avoids locking, and makes implementing Use getrandom on FreeBSD #35 easier.

The remaining uses of lazy_static can be eliminated by just using a combination of std::sync::Once and static mut in a follow up PR (see #52).

[josephlr]

This is based on @newpavlov's idea here: #50 (comment), it just uses AtomicBool instead of AtomicUsize, and encapsulates the complexity in a struct in a separate file.

[newpavlov]

Why do we need to bother with the LazyBool abstraction? I think it will be fine to simply use code in the issue, it's more efficient and not that complex in comparison. (ideally it would use AtomicU8, but it will require bumping MSRV for UEFI and SGX targets)

UPD: Ok, sorry I haven't read the description, only briefly looked at the code.

[josephlr]

Why do we need to bother with the LazyBool abstraction?

Having the abstraction is crucial if we want to use this logic in multiple places (like this PR does).

[josephlr]

I've update the description for this. Now there are two lock-free helpers LazyBool and Weak, both using an AtomicUsize to implement the lazy initialization.

[dhardy]

Wrong explanation. Probably you know, but the reason is that we only have a single shared variable.

[dhardy]

https://llvm.org/docs/Atomics.html#atomic-orderings

[josephlr]

Oh duh, I was overthinking my explanation. Fixed.

I also renamed the lazy module to be util_libc as I'm anticipating we will need more libc based utility functions.