host-cpu-c-abi: Update comment, working around FreeBSD breakage

[Bruno Haible]

For many years, the standard way to determine the endianness on powerpc64
at compile-time was

  - either to test the macros _BIG_ENDIAN or _LITTLE_ENDIAN (predefined
    by the compiler),

  - or to test for the ELFv2 ABI: _CALL_ELF == 2.

The port of FreeBSD 14 to powerpc64 breaks *both* ways:

  - When <sys/_endian.h> happens to be included, for example through
    <sys/types.h>, both _BIG_ENDIAN and _LITTLE_ENDIAN get defined!
    This is voluntary breakage, see
    
<https://cgit.freebsd.org/src/commit/?id=6f23df1ae7f6ec3cd06a15c87f0aaaa770b2941b>.

  - FreeBSD uses the ELFv2 ABI also on big-endian powerpc64. Which is
    allowed by
    
https://ftp.rtems.org/pub/rtems/people/sebh/ABI64BitOpenPOWERv1.1_16July2015_pub.pdf
    — but still it's the first OS to do so.


2024-08-23  Bruno Haible  <bruno@clisp.org>

        host-cpu-c-abi: Update comment, working around FreeBSD breakage.
        * m4/host-cpu-c-abi.m4: For powerpc64, recommend to test
        __BIG_ENDIAN__ or __LITTLE_ENDIAN__, not _BIG_ENDIAN or _LITTLE_ENDIAN,
        as FreeBSD 14 breaks the latter.

diff --git a/m4/host-cpu-c-abi.m4 b/m4/host-cpu-c-abi.m4
index 6053930288..80a8366855 100644
--- a/m4/host-cpu-c-abi.m4
+++ b/m4/host-cpu-c-abi.m4
@@ -1,5 +1,5 @@
 # host-cpu-c-abi.m4
-# serial 17
+# serial 18
 dnl Copyright (C) 2002-2024 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -36,7 +36,7 @@
 dnl   determine the endianness through preprocessor symbols:
 dnl   - 'arm': test __ARMEL__.
 dnl   - 'mips', 'mipsn32', 'mips64': test _MIPSEB vs. _MIPSEL.
-dnl   - 'powerpc64': test _BIG_ENDIAN vs. _LITTLE_ENDIAN.
+dnl   - 'powerpc64': test __BIG_ENDIAN__ vs. __LITTLE_ENDIAN__.
 dnl * The same name 'i386' is used for CPUs of type i386, i486, i586
 dnl   (Pentium), AMD K7, Pentium II, Pentium IV, etc., because
 dnl   - Instructions that do not exist on all of these CPUs (cmpxchg,