Fix GDB build: ELF support check & -lzstd

Fix GDB build: ELF support check & -lzstd

GDB fails to build for me, on Ubuntu 20.04.  I get:

 ...
   CXXLD  gdb
 /usr/bin/ld: linux-tdep.o: in function `linux_corefile_thread(thread_info*, linux_corefile_thread_data*)':
 /home/pedro/gdb/binutils-gdb/src/gdb/linux-tdep.c:1831: undefined reference to `gcore_elf_build_thread_register_notes(gdbarch*, thread_info*, gdb_signal, bfd*, std::unique_ptr<char, gdb::xfree_deleter<char> >*, int*)'
 /usr/bin/ld: linux-tdep.o: in function `linux_make_corefile_notes(gdbarch*, bfd*, int*)':
 /home/pedro/gdb/binutils-gdb/src/gdb/linux-tdep.c:2117: undefined reference to `gcore_elf_make_tdesc_note(bfd*, std::unique_ptr<char, gdb::xfree_deleter<char> >*, int*)'
 collect2: error: ld returned 1 exit status
 make[2]: *** [Makefile:2149: gdb] Error 1
 make[2]: Leaving directory '/home/pedro/gdb/binutils-gdb/build/gdb'
 make[1]: *** [Makefile:11847: all-gdb] Error 2
 make[1]: Leaving directory '/home/pedro/gdb/binutils-gdb/build'
 make: *** [Makefile:1004: all] Error 2

Those undefined functions exist in gdb/gcore-elf.c, which is only
included in the build if GDB's configure thinks that the target you're
configuring for is an ELF target.  GDB's configure thinks my system
isn't ELF, which is incorrect.

For the ELF support check, gdb/config.log shows:

 configure:17387: checking for ELF support in BFD
 configure:17407: gcc -o conftest -I/home/pedro/gdb/binutils-gdb/src/gdb/../include -I../bfd -I/home/pedro/gdb/binutils-gdb/src/gdb/../bfd -g3 -O0      -L../bfd -L../libiberty  -lzstd   conftest.c -lbfd -liberty -lz  -lncursesw -lm -ldl  >&5
 /usr/bin/ld: ../bfd/libbfd.a(compress.o): in function `decompress_contents':
 /home/pedro/gdb/binutils-gdb/src/bfd/compress.c:42: undefined reference to `ZSTD_decompress'
 /usr/bin/ld: /home/pedro/gdb/binutils-gdb/src/bfd/compress.c:44: undefined reference to `ZSTD_isError'
 /usr/bin/ld: ../bfd/libbfd.a(compress.o): in function `bfd_compress_section_contents':
 /home/pedro/gdb/binutils-gdb/src/bfd/compress.c:195: undefined reference to `ZSTD_compress'
 /usr/bin/ld: /home/pedro/gdb/binutils-gdb/src/bfd/compress.c:198: undefined reference to `ZSTD_isError'
 collect2: error: ld returned 1 exit status
 configure:17407: $? = 1
 ...
 configure:17417: result: no

Note how above, in the gcc command line, "-lzstd" appears before
"-lbfd".  That explain the link failure.  It should appear after, like
-lz does.

This commit fixes it, by moving ZSTD_LIBS from LDFLAGS to LIBS, next
to -lz, in GDB_AC_CHECK_BFD, and regenerating gdb/configure.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29630
Change-Id: I1f4128dde634e8ea04c9002904f1005a8b3a6863