std.posix: panic on unexpected error in munmap

This is to help diagnose #25498. We can't use `unexpectedErrno` here,
because `std.posix.munmap` is infallible. So, when the flag is set to
report unexpected errnos, we just call `std.debug.panic` to provide
details instead of doing `unreachable`.

Pushing straight to master after running checks locally; there's no
point waiting for CI on the PR just for this.

lib/std/posix.zig

@@ -4991,7 +4991,9 @@ pub fn munmap(memory: []align(page_size_min) const u8) void {
         .SUCCESS => return,
         .INVAL => unreachable, // Invalid parameters.
         .NOMEM => unreachable, // Attempted to unmap a region in the middle of an existing mapping.
-        else => unreachable,
+        else => |e| if (unexpected_error_tracing) {
+            std.debug.panic("unexpected errno: {d} ({t})", .{ @intFromEnum(e), e });
+        } else unreachable,
     }
 }