Allow atomic operations up to 32 bits

The ARMv5te platform does not have instruction-level support for atomics, however the kernel provides [user space helpers](https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt) which can be used to perform atomic operations. When linked with `libc`, the atomic symbols needed by Rust will be provided, rather than CPU level intrinsics.

As this target is specifically `linux` and `gnueabi`, it is reasonable to assume the Linux Kernel and libc will be available for the target. There is a large performance penalty, as we are not using CPU level intrinsics, however this penalty is likely preferable to not having the target at all.

I have used this change in a custom target (along with `xargo`) to build `std`, as well as a number of higher level crates.

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