add macOS handling for totalSystemMemory

[imomaliev]

Hi, I just started learing zig. I was trying to fetch total system memory on macOS and noticed that totalSystemMemory does not support it.

This PR adds it.

I believe there are a few issues with totalSystemMemory for .freebsd and std.posix.sysctlbynameZ, but I do not know what the correct approach to take here

1. totalSystemMemory for .freebsd returns usize instead of u64, I believe this should be fixed
2. totalSystemMemory for .freebsd does not handle all the errors from std.posix.sysctlbynameZ.
3. Last but not least, std.posix.sysctlbynameZ never returns NameTooLong. So either we should change the return type, or, as @andrewrk did in 3640303 we should add name_len handling. I did not find any mention of ENAMETOOLONG in manpages for freebsd/posix. So I am having a hard time understanding which way is correct here.

Just for the context I know that there were discussions in #16350 that resulted in the rollback of patches that already implemented this functionality before. Please let me know what should be done to meet contributing standards to avoid same fate as previous contributor 🙂

I am ready to provide patches for 1 and 2 in this PR or create separate one

Thanks!

[alexrp]

totalSystemMemory for .freebsd returns usize instead of u64, I believe this should be fixed

u64 looks right to me.

totalSystemMemory for .freebsd does not handle all the errors from std.posix.sysctlbynameZ.

This doesn't matter too much since I'll get to it when I do #6389 anyway. But yes, it probably should.

Last but not least, std.posix.sysctlbynameZ never returns NameTooLong. So either we should change the return type, or, as @andrewrk did in 3640303 we should add name_len handling. I did not find any mention of ENAMETOOLONG in manpages for freebsd/posix. So I am having a hard time understanding which way is correct here.

I think just remove the dead NameTooLong prong.

[imomaliev]

@alexrp Thanks. Fixed 1, but decided to fix 3 in separate PR #24909

[alexrp]

The type of physmem should be changed to u64, making the cast unnecessary.