[libc][sched] Implement CPU_ZERO, CPU_ISSET, CPU_SET macros

[krishna2803]

This PR implements the following macros for sched.h:

• CPU_ZERO
• CPU_ISSET
• CPU_SET

Fixes #124642

[krishna2803]

@jhuber6 @lntue could you please review this

[jhuber6]

Does this need parens?

[krishna2803]

fixed in c94712f1

[lntue]

What does the standard say when these pointers are null?

[krishna2803]

these macros are glibc extensions, the man page itself doesn't define a way to deal with null pointers. i found an implementation of those in glibc's code and surprisingly even that doesn't handle null pointers. ref:

#define __CPU_ISSET_S(cpu, setsize, cpusetp) \
  (__extension__							      \
   ({ size_t __cpu = (cpu);						      \
      __cpu / 8 < (setsize)						      \
      ? ((((const __cpu_mask *) ((cpusetp)->__bits))[__CPUELT (__cpu)]	      \
	  & __CPUMASK (__cpu))) != 0					      \
      : 0; }))

for implementing these macros (CPU_ZERO et al), i looked at our existing implementation for CPU_COUNT ref:

llvm-project/libc/src/sched/linux/sched_getcpucount.cpp

Lines 19 to 26 in c94712f

	LLVM_LIBC_FUNCTION(int, __sched_getcpucount,
	(size_t cpuset_size, const cpu_set_t *mask)) {
	int result = 0;
	for (size_t i = 0; i < cpuset_size / sizeof(long); ++i) {
	result += __builtin_popcountl(mask->__mask[i]);
	}
	return result;
	}

and even that doesn't handle null pointers!

---

should i implement a nullptr guard for these?

[lntue]

Yes, can you add null check with https://github.com/llvm/llvm-project/blob/main/libc/src/__support/macros/null_check.h for these functions in this PR? Thanks,

[krishna2803]

done! 522c6697

[lntue]

Nit: Update the cmake dependency of these targets.

[krishna2803]

done! 9b0d165e