Use uint32 instead of uint (64-bit in Go on s390x) to store the return value of the TIOCGPTN syscall. This is to avoid the 32-bit value from being stored into a 64-bit buffer and get left-shifted by 32 when dereferencing, turning what should be e.g. /dev/pts/1 to /dev/pts/4294967296 on big-endian architectures such as s390x.

Special thanks to the explanation and a similar bug fix provided at containerd/console#51

Fixes the following TestScripts/pty error on s390x:

=== NAME  TestScripts/pty
    testscript.go:558: > [!linux] [!darwin] skip
        > [darwin] skip # https://go.dev/issue/61779
        > ttyin secretwords.txt
        > terminalprompt
        [failed to open TTY: open /dev/pts/21474836480: no such file or directory]
        FAIL: testdata/pty.txt:5: unexpected command failure

discovered by Debian autopkgtest CI for golang-github-rogpeppe-go-internal/1.12.0-1;
see log at https://ci.debian.net/packages/g/golang-github-rogpeppe-go-internal/testing/s390x/43022768/#L3047

(Hmm... I guess /dev/pts/21474836480 should have been /dev/pts/5, as 5 << 32 = 21474836480.)

Special thanks also to @kapouer for providing an interim workaround in golang-github-rogpeppe-go-internal/1.12.0-2 for Debian as 1.12.0-1 was blocked from entering "trixie (testing)" for 16 days due to test error on s390x.