add support for platforms without atomic ptr

[LinuxHeki]

Add support for platforms without support for atomic operations on pointers (such as thumbv6m-none-eabi).

[matklad]

Hm, this feels incorrect to me.

Consider threads A, B, C and the following sequence of events:

   A   |   B     |   C
       |         |
load 0 |         |
       | load 0  |
       | store 1 |
       |         | load 1
store 2|         |
       |         | load 2

Here, A and B race when initializing the inner, so C observes two different values of once cell, which violates the invariant that makes the whole construct a safe abstraction.

[LinuxHeki]

True... Does that mean this crate can't be ported platforms without atomic ptr?

[taiki-e]

platforms without atomic ptr

"critical-section" feature is a feature to use sync/race in such targets.

once_cell/Cargo.toml

Lines 48 to 51 in c48d3c2

	# Uses `critical-section` to implement `sync` and `race` modules. in
	# `#![no_std]` mode. Please read `critical-section` docs carefully
	# before enabling this feature.
	critical-section = ["dep:critical-section", "portable-atomic" ]