ppc64le-linux-gnu: Skip testing *printf functions

Ubuntu 26.04 transitioned PowerPC64LE from the IBM long double format to
IEEE binary128 [1], which mean a handful of variadic functions needed to
switch to a binary128-compatible version. These were available starting
with glibc 2.32 (2020-08) per the abilist [2], which is quite a bit
newer than what we say we support at [3] (2.17, 2012-12).

This explains test failures, so skip checking the functions per now. We
should migrate to the `__<name>ieee128` functions at some point, which
need not be related to our documented minimum, but there is no reason to
do it now since Rust doesn't even stably support either IBM `long
double` or binary128.

Demo on Ubuntu 24.04:

    $ cat demo.c
    #include <stdio.h>
    #include <syslog.h>

    void demo() {
            char buf1[20] = { 0 };
            char buf2[20] = { 0 };
            printf("Hello, world! %d\n", 1);
            fprintf(stderr, "Hello, world! %d\n", 2);
            sprintf(buf1, "Hello, world! %d\n", 3);
            snprintf(buf2, 20, "Hello, world! %d\n", 4);
            syslog(0, "Hello, world! %d\n", 5);
    }
    $ powerpc64le-linux-gnu-gcc demo.c -c
    $ powerpc64le-linux-gnu-nm demo.o
                     U .TOC.
                     U __stack_chk_fail
    0000000000000000 T demo
                     U fprintf
                     U printf
                     U snprintf
                     U sprintf
                     U stderr
                     U syslog

And the same demo on Ubuntu 26.04:

    $ cat demo.c
    #include <stdio.h>
    #include <syslog.h>

    void demo() {
            char buf1[20] = { 0 };
            char buf2[20] = { 0 };
            printf("Hello, world! %d\n", 1);
            fprintf(stderr, "Hello, world! %d\n", 2);
            sprintf(buf1, "Hello, world! %d\n", 3);
            snprintf(buf2, 20, "Hello, world! %d\n", 4);
            syslog(0, "Hello, world! %d\n", 5);
    }
    $ powerpc64le-linux-gnu-gcc demo.c -c
    $ powerpc64le-linux-gnu-nm demo.o
                     U .TOC.
                     U __fprintfieee128
                     U __printfieee128
                     U __snprintfieee128
                     U __sprintfieee128
                     U __stack_chk_fail
                     U __syslogieee128
    0000000000000000 T demo
                     U stderr

[1]: https://bugs.launchpad.net/ubuntu/+bug/2132257
[2]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/powerpc/powerpc64/le/libc.abilist;h=65d78e50760b5d6b1a9cb39016a7bf8064224794;hb=HEAD#l2519
[3]: https://doc.rust-lang.org/1.96.0/rustc/platform-support.html

Author: tgross35

libc-test/build.rs

@@ -3805,6 +3805,8 @@ fn test_linux(target: &str) {
     let i686 = target.contains("i686");
     let ppc = target.contains("powerpc");
     let ppc64 = target.contains("powerpc64");
+    let ppc64le = target.contains("powerpc64le");
+    let _ppc64be = target.contains("powerpc64-");
     let ppc32 = ppc && !ppc64;
     let s390x = target.contains("s390x");
     let sparc = target.contains("sparc");
@@ -4950,6 +4952,10 @@ fn test_linux(target: &str) {
             // FIXME(linux): function pointers changed since Ubuntu 23.10
             "strtol" | "strtoll" | "strtoul" | "strtoull" | "fscanf" | "scanf" | "sscanf" => true,
 
+            // FIXME(ppc): function pointers changed in Ubuntu 26.04 to support IEEE binary128
+            // `long double`. We should update at some point but there is no hurry.
+            "printf" | "fprintf" | "sprintf" | "snprintf" | "syslog" if gnu && ppc64le => true,
+
             _ => false,
         }
     });