linux: For better compatibility, provide compatible endian.h

Add endian.h. This includes sys/endian.h and then adds extra defines
that glibc defines with double underscores for our
_{BIG,BYTE,LITTLE,PDP}_ENDIAN macros. We also define __FLOAT_WORD_ORDER
to be the same as _BYTE_ENDIAN since FreeBSD doesn't currently define
this, and the default with glibc is exactly this for our platforms.
Move common parts of endian.h and sys/endian.h into sys/_endian.h
to limit namespace pollution from endian.h

All this gives us good compatibility with Linux. There may be one or two
upstreams that haven't integrated the patches I tried to send up.

There are some minor differences:
	o The extra glibc macros are not defined. These are all
	  controlled with either __ at the start, or only defined
	  when glibc is being built. We also don't define macros
	  that are used internally in glibc that would pollute
	  the namespace.
	o For complete compatibility, this change must also be
	  paired with providing a glibc-compatible byteswap.h.

Sponsored by:		Netflix
Reviewed by:		mhorne, markj, jhb
Differential Revision:	https://reviews.freebsd.org/D31962

include/Makefile

@@ -14,8 +14,8 @@ SUBDIR+=	i386
 SUBDIR_PARALLEL=
 INCS=	a.out.h ar.h assert.h bitstring.h complex.h cpio.h _ctype.h ctype.h \
 	db.h \
-	dirent.h dlfcn.h elf.h elf-hints.h err.h fmtmsg.h fnmatch.h fstab.h \
-	fts.h ftw.h getopt.h glob.h grp.h \
+	dirent.h dlfcn.h elf.h elf-hints.h endian.h err.h fmtmsg.h fnmatch.h \
+	fstab.h fts.h ftw.h getopt.h glob.h grp.h \
 	ieeefp.h ifaddrs.h \
 	inttypes.h iso646.h kenv.h langinfo.h libgen.h limits.h link.h \
 	locale.h malloc.h malloc_np.h memory.h monetary.h mpool.h mqueue.h \

include/endian.h

@@ -0,0 +1,43 @@
+/*-
+ * Copyright (c) 2021 M. Warner Losh <imp@FreeBSD.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+/*
+ * A mostly Linux/glibc-compatible endian.h
+ */
+
+#ifndef _ENDIAN_H_
+#define _ENDIAN_H_
+
+/*
+ * FreeBSD's sys/_endian.h is very close to the interface provided on Linux by
+ * glibc's endian.h.
+ */
+#include <sys/_endian.h>
+
+/*
+ * glibc uses double underscore for these symbols. Define these unconditionally.
+ * The compiler defines __BYTE_ORDER__ these days, so we don't do anything
+ * with that since sys/endian.h defines _BYTE_ORDER based on it.
+ */
+#define __BIG_ENDIAN		_BIG_ENDIAN
+#define __BYTE_ORDER		_BYTE_ORDER
+#define __LITTLE_ENDIAN		_LITTLE_ENDIAN
+#define __PDP_ENDIAN		_PDP_ENDIAN
+
+/*
+ * FreeBSD's sys/endian.h and machine/endian.h doesn't define a separate
+ * byte order for floats. Use the host non-float byte order.
+ */
+#define __FLOAT_WORD_ORDER	_BYTE_ORDER
+
+/*
+ * We don't define BIG_ENDI, LITTLE_ENDI, HIGH_HALF and LOW_HALF macros that
+ * glibc's endian.h defines since those appear to be internal to internal to
+ * glibc. We also don't try to emulate the various helper macros that glibc
+ * uses to limit namespace visibility.
+ */
+
+#endif /* _ENDIAN_H_ */