macho linking uses incorrect value for headerpad_max_install_names

[andrewrk]

This value, std.os.PATH_MAX applies to the host target. However the linker is supposed to be linking for the target given to the compiler at runtime.

zig/src/link/MachO.zig

Line 3540 in 3f577f0

const name_len = if (assume_max_path_len) std.os.PATH_MAX else std.mem.len(name) + 1;

This bug is revealed clearly when compiling for WASI which has no std.os.PATH_MAX value, yet should still be able to cross-compile macos binaries:

$ stage3/bin/zig build -p wasi -Dskip-install-lib-files -Dtarget=wasm32-wasi
/home/andy/Downloads/zig/lib/std/os.zig:110:28: error: root struct of file 'os.wasi' has no member named 'PATH_MAX'
pub const PATH_MAX = system.PATH_MAX;
                     ~~~~~~^~~~~~~~~
referenced by:
    calcInstallNameLen: /home/andy/Downloads/zig/src/link/MachO.zig:3540:53
    calcLCsSize: /home/andy/Downloads/zig/src/link/MachO.zig:3577:24
    remaining reference traces hidden; use '-freference-trace' to see all reference traces

In theory we should be able to use instead something like std.os.darwin.PATH_MAX however the std lib is organized in a way that only makes that work if the host matches.

A quickfix instead would look something like this:

--- a/src/link/MachO.zig
+++ b/src/link/MachO.zig
@@ -3537,7 +3537,8 @@ pub fn populateMissingMetadata(self: *MachO) !void {
 }
 
 inline fn calcInstallNameLen(cmd_size: u64, name: []const u8, assume_max_path_len: bool) u64 {
-    const name_len = if (assume_max_path_len) std.os.PATH_MAX else std.mem.len(name) + 1;
+    const darwin_path_max = 1024;
+    const name_len = if (assume_max_path_len) darwin_path_max else std.mem.len(name) + 1;
     return mem.alignForwardGeneric(u64, cmd_size + name_len, @alignOf(u64));
 }

[andrewrk]

Moving to 0.10.0 milestone so I can say in the release notes that Zig compiles for WASI now.