Package fetch: add executable detection for Mach-O file headers

[fabioarnold]

When trying to execute a binary fetched from a package I ran into this issue: #21044:

the expected behavior from the package manager is to ignore permissions from the zip file and set the execute bit based on ELF header and shebang line alone (see #17463)

Since this has been implemented for ELF already this change adds support for Mach-O.

[owenbrooks]

Great! Would be good to add std.macho.FAT_MAGIC and std.macho.FAT_MAGIC_64 to support multi-architecture binaries too.

[andrewrk]

Great, thanks! This is breaking right? Since it will cause the hash to change on macOS for any executable files.

[fabioarnold]

Well thanks for reviewing and merging! :)

The way I understand the code is the package is fetched and extracted while files are created with the default mode (ModeMode.ignore in tar.zig), the hash is computed before ultimately the executable bits are set depending on the file type.

So this should be a non-breaking change. I checked this by deleting $HOME/.cache/zig and building my project each with a previous version of Zig and current master with this change. I didn't have to update my dependency's hash as was expected.