libctf: make it compile for old glibc

libctf: make it compile for old glibc

With a glibc before 2.9 (such as 2.8), there's <endian.h> but no
htole64 or le64toh, so you get, compiling binutils for any target:

libtool: link: gcc -W -Wall -Wstrict-prototypes -Wmissing-prototypes \
-Wshadow -Werror -I/x/binutils/../zlib -g -O2 -o objdump \
objdump.o dwarf.o prdbg.o rddbg.o debug.o stabs.o rdcoff.o \
bucomm.o version.o filemode.o elfcomm.o  ../opcodes/.libs/libopcodes.a \
../libctf/libctf.a ../bfd/.libs/libbfd.a -L/x/obj/b/zlib -lz ../libiberty/libiberty.a -ldl
../libctf/libctf.a(ctf-archive.o): In function `ctf_archive_raw_iter_internal':
/x/src/libctf/ctf-archive.c:543: undefined reference to `le64toh'
/x/src/libctf/ctf-archive.c:550: undefined reference to `le64toh'
/x/src/libctf/ctf-archive.c:551: undefined reference to `le64toh'
/x/src/libctf/ctf-archive.c:551: undefined reference to `le64toh'
/x/src/libctf/ctf-archive.c:554: undefined reference to `le64toh'
../libctf/libctf.a(ctf-archive.o):/x/src/libctf/ctf-archive.c:545: more undefined references to `le64toh' follow
(etc)

Also, I see no bswap_identity_64 *anywhere* except in libctf/swap.h
(including current glibc) and I don't think calling an "identity"-
function is better than just plain "#define foo(x) (x)" anyway.
(Where does the idea of a bytestap.h bswap_identity_64 come from?)

Speaking of that, I should mention that I instrumented the condition
to observe that the WORDS_BIGENDIAN case passes too for a presumed
big-endian target and glibc-2.8: there is a bswap_64 present for that
version.  Curiously, no test-case regressed with that instrumentation.

For the record, constructing binary blobs using text source to run
tests on, can be done by linking to --oformat binary (with most ELF
targets), but I guess that's seen as unnecessary roundabout perhaps
checking in binary files in the test-suite would be ok these days.
[...]

[nca: trimmed commit log slightly, updated changelog]

v5: fix tabdamage.

libctf/
	* ctf-endian.h: Don't assume htole64 and le64toh are always
	present if HAVE_ENDIAN_H; also check if htole64 is defined.
	[!WORDS_BIGENDIAN] (htole64, le64toh): Define as identity,
	not bswap_identity_64.