Fix copy-pasto, allocate objfile_per_bfd_storage with obstack_new

Fix copy-pasto, allocate objfile_per_bfd_storage with obstack_new

I realized after pushing that I made a copy-pasto, I had:

  # define HAVE_IS_TRIVIALLY_COPYABLE 1

instead of

  # define HAVE_IS_TRIVIALLY_CONSTRUCTIBLE 1

with the consequence that IsMallocable was always std::true_type (and
was therefore not enforcing anything).  Fixing that mistake triggered a
build failure:

/home/simark/src/binutils-gdb/gdb/objfiles.c:150:12:   required from here
/home/simark/src/binutils-gdb/gdb/common/poison.h:228:3: error: static assertion failed: Trying to use XOBNEW with a non-POD data type.

I am not sure why I did not see this when I originally wrote the patch
(but I saw and fixed other failures).  In any case, I swapped XOBNEW
with obstack_new to get rid of it.

Regtested on the buildbot.

gdb/ChangeLog:

	* common/traits.h (HAVE_IS_TRIVIALLY_COPYABLE): Rename the wrong
	instance to...
	(HAVE_IS_TRIVIALLY_CONSTRUCTIBLE): ... this.
	* objfiles.c (get_objfile_bfd_data): Allocate
	objfile_per_bfd_storage with obstack_new when allocating on
	obstack.