Utils and libraries Revert <<Use getentropy rather than /dev/urandom on macOS>> #15100

This reverts commit ee2d10a (#10301).

I've checked sys/random.h (/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/sys/random.h) and it didn't have attribute((weak_import)). It was:

__attribute__((availability(macosx,introduced=10.12))) __attribute__((availability(ios,introduced=10.0))) __attribute__((availability(tvos,introduced=10.0))) __attribute__((availability(watchos,introduced=3.0)))
int getentropy(void* buffer, size_t size);

for Clang. For GCC:

int getentropy(void* buffer, size_t size);

The unpreprocessed version (so that you can see that it has no intention to __attribute__((weak_import))):

__BEGIN_DECLS
__OSX_AVAILABLE(10.12) __IOS_AVAILABLE(10.0) __TVOS_AVAILABLE(10.0) __WATCHOS_AVAILABLE(3.0)
int getentropy(void* buffer, size_t size);
__END_DECLS

To double-check, I decided to compile this on macOS 10.14 with GCC 8.2:

#include <stdio.h>
#include <sys/random.h>

int main(void) {
	unsigned char ent32[10];
	if (getentropy) puts("Have it!");
	else puts("Don't have it!");
	printf("%d\n", getentropy(ent32, 5));
	return 0;
}

When I ran it on 10.10, it said:

Have it!
dyld: lazy symbol binding failed: Symbol not found: _getentropy
  Referenced from: /Users/me/Downloads/a.out
  Expected in: /usr/lib/libSystem.B.dylib

dyld: Symbol not found: _getentropy
  Referenced from: /Users/me/Downloads/a.out
  Expected in: /usr/lib/libSystem.B.dylib

Trace/BPT trap: 5

My suggestion is, if we're supporting 10.11 or lower, we should avoid taking risks and continue with /dev/urandom.

cc @jameshilliard

What happens if you change this:

#include <stdio.h>
#include <sys/random.h>

int main(void) {
  unsigned char ent32[10];
  if (getentropy) puts("Have it!");
  else puts("Don't have it!");
  printf("%d\n", getentropy(ent32, 5));
  return 0;
}

to:

#include <stdio.h>
#include <sys/random.h>

int main(void) {
	unsigned char ent32[10];
	if (&getentropy != nullptr) puts("Have it!");
	else puts("Don't have it!");
	printf("%d\n", getentropy(ent32, 5));
	return 0;
}

Possible fix in #15103 if this is verified to be an issue.

That code also failed. This one worked (printed "Don't have it!" and then crashed, as expected):

#include <stdio.h>

extern int getentropy(void *buffer, size_t size) __attribute__((weak_import));

int main(void) {
	unsigned char ent32[10];
	if (&getentropy != NULL) puts("Have it!");
	else puts("Don't have it!");
	printf("%d\n", getentropy(ent32, 5));
	return 0;
}

But note that the code above does not build on old macOS where there is no getentropy, due to linker error.

Ok, updated my pull request with that as the fix.