gdb: CTF support

This patch adds the CTF (Compact Ansi-C Type Format) support in gdb.
Two submissions on which this gdb work depends were posted earlier
in May:

 * On the binutils mailing list - adding libctf which creates, updates,
   reads, and manipulates the CTF data.
 * On the gcc mailing list - expanding gcc to directly emit the CFT data
   with a new command line option -gt.

CTF is a reduced form of debugging information whose main purpose is to
describe the type of C entities such as structures, unions, typedefs and
function arguments at the global scope only. It does not contain debug
information about source lines, location expressions, or local variables.
For more information on CTF, see the documentation in the libdtrace-ctf
source tree, available here:

<https://raw.githubusercontent.com/oracle/libdtrace-ctf/master/doc/ctf-format>.

This patch expands struct elfinfo by adding the .ctf section, which
contains CTF debugging info, and modifies elf_symfile_read() to read it.
If both DWARF and CTF exist in a program, only DWARF will be read. CTF data
will be read only when there is no DWARF. The two-stage symbolic reading
and setting strategy, partial and full, was used.

File ctfread.c contains functions to transform CTF data into gdb's internal
symbol table structures by iterately reading entries from CTF sections
of "data objects", "function info", "variable info", and "data types"
when setting up either partial or full symbol table. If the ELF symbol table
is available, e.g. not stripped, the CTF reader will associate the found
type information with these symbol entries. Due to the proximity between DWARF
and CTF (CTF being a much simplified subset of DWARF), some DWARF implementation
was reused to support CTF.

Test cases ctf-constvars.exp, ctf-cvexpr.exp, ctf-ptype.exp, and ctf-whatis.exp
have been added to verify the correctness of this support.

This patch has missing features and limitations which we will add and
address in the future patches.

gdb/ChangeLog
+2019-10-07  Weimin Pan  <weimin.pan@oracle.com>
+
+       * gdb/ctfread.c: New file.
+       * gdb/ctfread.h: New file.
+       * gdb/elfread.c: Include ctfread.h.
+       (struct elfinfo text_p): New member ctfsect.
+       (elf_locate_sections): Mark CTF section.
+       (elf_symfile_read): Call elfctf_build_psymtabs.
+       * gdb/Makefile.in (LIBCTF): Add.
+       (CLIBS): Use it.
+       (CDEPS): Likewise.
+       (DIST): Add ctfread.c.
+       * Makefile.def (dependencies): Add all-libctf to all-gdb
+       * Makefile.in: Add "all-gdb: maybe-all-libctf"
+
gdb/testsuite/ChangeLog
+2019-10-07  Weimin Pan  <weimin.pan@oracle.com>
+
+       * gdb.base/ctf-whatis.exp: New file.
+       * gdb.base/ctf-whatis.c: New file.
+       * gdb.base/ctf-ptype.exp: New file.
+       * gdb.base/ctf-ptype.c: New file.
+       * gdb.base/ctf-constvars.exp: New file.
+       * gdb.base/ctf-constvars.c: New file.
+       * gdb.base/ctf-cvexpr.exp: New file.
+

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