Minor fixes for macOS and powerpc

[barracuda156]

A few fixes to allow the build on older macOS versions.

1. The build hardcodes a few assumptions for macOS which may not hold. I cannot verify this on 10.7, so the threshold is based on documentation. I can say for sure that on 10.6 these are necessary.
2. Including <sys/random.h> may need <sys/types.h>, otherwise the build breaks on system headers, complaining that u_int has not been declared.
3. Unbreak powerpc module; this basically restores the code from Botan v2 with minor changes to fit it here. (I include OpenBSD simply because it is there in Botan code; I have no idea whether OpenBSD still needs a dedicated code, and can drop it from macros, if needed.)

[coveralls]

coverage: 90.56% (-0.4%) from 90.975%
when pulling 977ff7e on barracuda156:apple
into 5fbcc7d on randombit:master.

[randombit]

CPUID changes look fine

[barracuda156]

CPUID changes look fine

Thank you, I will make changes according to your suggestions today and rebase the PR.

[barracuda156]

@randombit I have dropped two commits, as advised, switched to __has_include and rebased.

[randombit]

@barracuda156 Format checks are failing, you should be able to just apply the patch printed in the failing job

[barracuda156]

@randombit I will check that, thanks.

[randombit]

@barracuda156 LGTM please squash and then this can merge