fortify: Capture __bos() results in const temp vars

In two recent run-time memcpy() bound checking bug reports (NFS[1] and
JFS[2]), the _detection_ was working correctly (in the sense that the
requested copy size was larger than the destination field size), but
the _warning text_ was showing the destination field size as SIZE_MAX
("unknown size"). This should be impossible, since the detection function
will explicitly give up if the destination field size is unknown. For
example, the JFS warning was:

  memcpy: detected field-spanning write (size 132) of single field "ip->i_link" at fs/jfs/namei.c:950 (size 18446744073709551615)

Other cases of this warning (e.g.[3]) have reported correctly,
and the reproducer only happens under GCC (at least 10.2 and 12.1),
so this currently appears to be a GCC bug. Explicitly capturing the
__builtin_object_size() results in const temporary variables fixes the
report. For example, the JFS reproducer now correctly reports the field
size (128):

  memcpy: detected field-spanning write (size 132) of single field "ip->i_link" at fs/jfs/namei.c:950 (size 128)

Examination of the .text delta (which is otherwise identical), shows
the literal value used in the report changing:

-     mov    $0xffffffffffffffff,%rcx
+     mov    $0x80,%ecx

[1] https://lore.kernel.org/lkml/Y0zEzZwhOxTDcBTB@codemonkey.org.uk/
[2] https://syzkaller.appspot.com/bug?id=23d613df5259b977dac1696bec77f61a85890e3d
[3] https://lore.kernel.org/all/202210110948.26b43120-yujie.liu@intel.com/

Cc: "Dr. David Alan Gilbert" <linux@treblig.org>
Cc: llvm@lists.linux.dev
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>