libctf, elfcpp, gold: do not assume that <byteswap.h> contains bswap_*

libctf, elfcpp, gold: do not assume that <byteswap.h> contains bswap_*

At least one C library (uclibc-ng) defines some of these only when
the compiler is GCC.  We might as well test for all three cases and
handle any of them being missing.

Very similar code exists in libctf and split between elfcpp and gold:
fix both.

(Also sync up elfcpp with a change made to libctf swap.h a few months
ago: since there is no out-of-line definition of the bswap replacements,
they should be declared static inline, not just inline, to prevent the
linker generating out-of-line references to them.)

	PR libctf/25120
libctf/
	* configure.ac: Check for bswap_16, bswap_32, and bswap_64 decls.
	* swap.h (bswap_16): Do not assume that presence of <byteswap.h>
	means this is declared.
	(bswap_32): Likewise.
	(bswap_64): Likewise.
	(bswap_identity_64): Remove, unused.
	* configure: Regenerated.
	* config.h.in: Likewise.
gold/
	* configure.ac: Check for bswap_16, bswap_32, and bswap_64 decls.
	* configure: Regenerated.
	* config.h.in: Likewise.
elfcpp/
	* elfcpp_swap.h (bswap_16): Do not assume that presence of
	<byteswap.h> means this is declared.  Make static inline, matching
	recent change to libctf, since there is no non-inline definition
	of these functions.
	(bswap_32): Likewise.
	(bswap_64): Likewise.