x86: set CONFIG_USE_PRIVATE_LIBGCC to y

The motivation of this commit is to change CONFIG_USE_PRIVATE_LIBGCC
to a boolean macro so we can move it to Kconfig.

In the current implementation, there are two forms of syntax
for this macro:

  - CONFIG_USE_PRIVATE_LIBGCC=y
  - CONFIG_USE_PRIVATE_LIBGCC=path/to/private/libgcc

The latter is only used by x86 architecture.
With a little bit refactoring, it can be converted to the former.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Tested-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>