gdb: initialize ctf_context::builder in create_partial_symtab

gdb: initialize ctf_context::builder in create_partial_symtab

I built a random project with -gctf, in order to test the CTF support in
GDB.  With my ASan/UBSan/etc-enabled build of GDB, I get:

    $ ./gdb --data-directory=data-directory /tmp/babeltrace-ctf/src/lib/.libs/libbabeltrace2.so.0.0.0
    ...
    Reading symbols from /tmp/babeltrace-ctf/src/lib/.libs/libbabeltrace2.so.0.0.0...
    /home/simark/src/binutils-gdb/gdb/ctfread.c:1545:31: runtime error: member call on misaligned address 0xbebebebebebebebe for type 'struct buildsym_compunit', which requires 8 byte alignment
    0xbebebebebebebebe: note: pointer points here

The 0xbebebebebebebebe value is a sign that the ctf_context::builder
field is uninitialized.  The problem probably goes under the radar if
the field happens to be zero-initialized, because ctf_start_archive
contains this code:

  if (ccx->builder == nullptr)
    {
      ccx->builder = new buildsym_compunit (of,
		      of->original_name, nullptr, language_c, 0);

If the field was zero-initialized (by chance), this will create a new
buildsym_compunit.  But if the field was purposely filled with random
bytes by one of the sanitizers, we won't create a buildsym_compunit here
and we'll continue with ccx->builder equal to 0xbebebebebebebebe.

Fix this the easy way by initializing ccx->builder where the other
ctf_context fields are initialized (yeah, this code could be made nicer
C++, but I am going for the obvious fix here).

With this patch, this passes cleanly on my system:

  $ make check TESTS="gdb.ctf/*.exp" RUNTESTFLAGS="CC_FOR_TARGET=/opt/gcc/git/bin/gcc"
  # of expected passes            40

... where /opt/gcc/git/bin/gcc is a gcc with CTF support, given my
system gcc does not have it.

Change-Id: Idea1b0cf3e3708b72ecb16b1b60222439160f9b9