Add ELFCOMPRESS_ZSTD ELF compression constant

ELFCOMPRESS_ZSTD indicates that an ELF section is compressed with zstd.
It is the second compression type, after the existing ELFCOMPRESS_ZLIB.
Zstd generally provides a compelling tradeoff of speed and compression
(other algorithms may compress slightly better but take a lot longer,
or run faster but do not compress nearly as well).

See https://groups.google.com/g/generic-abi/c/satyPkuMisk for details.
ELFCOMPRESS_ZSTD will be supported in a future Clang/LLVM update.  ELF
Tool Chain tools also need updating.

Reviewed by:	Fangrui Song
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

share/man/man5/elf.5

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 27, 2021
+.Dd July 25, 2022
 .Dt ELF 5
 .Os
 .Sh NAME
@@ -1103,6 +1103,10 @@ A value of
 .Dv ELFCOMPRESS_ZLIB
 indicates that the data is compressed using
 .Xr zlib 3 .
+A value of
+.Dv ELFCOMPRESS_ZSTD
+indicates that the data is compressed using
+Zstandard.
 .It Dv ch_size
 The size, in bytes, of the uncompressed section data.
 This corresponds to the

sys/sys/elf_common.h

@@ -944,6 +944,7 @@ typedef struct {
 
 /* Values for ch_type (compressed section headers). */
 #define	ELFCOMPRESS_ZLIB	1	/* ZLIB/DEFLATE */
+#define	ELFCOMPRESS_ZSTD	2	/* Zstandard */
 #define	ELFCOMPRESS_LOOS	0x60000000	/* OS-specific */
 #define	ELFCOMPRESS_HIOS	0x6fffffff
 #define	ELFCOMPRESS_LOPROC	0x70000000	/* Processor-specific */