Add Linux/macOS compatibility system headers to tools/build/cross-build

[arichardson]

Summary

These headers are required in order to build the bootstrap tools on macOS
and Linux. A follow-up commit will add implementations of functions that
don't exist on those operating systems to -legacy when bootstrapping.

[arichardson]

rebased, should be ready for review now

[brooks]

A few whitespace and comment formatting nits (not every instance marked), but generally looks fine.

tools/build/cross-build/include/common/machine/atomic.h

45

probably remove this line

tools/build/cross-build/include/linux/stdlib.h

61

I'd tend to pick one indent level for all the functions.

tools/build/cross-build/include/linux/sys/param.h

43

/* on separate line

tools/build/cross-build/include/linux/sys/stat.h

40

Too many *s

44

Probably use C-style comment.

tools/build/cross-build/include/mac/stdlib.h

44

should pick one indentation style.

[arichardson]

tools/build/cross-build/include/common/machine/atomic.h

45

In fact I think this header is no longer needed. Will see if I can remove it.

[arichardson]

• fix C++ style commments
• Use clang-format to get consistent style

[brooks]

tools/build/cross-build/include/common/pwd.h

57

Normally return (1);

[arichardson]

• Add $FreeBSD$ and missing parens around return statements