kbuild: add CONFIG_ prefix to USE_PRIVATE_LIBGCC

Before this commit, USE_PRIVATE_LIBGCC was defined in
arch-specific config.mk and referenced in
arch/$(ARCH)/lib/Makefile.

We are not happy about parsing config.mk again and again.
We have to keep the same behavior with a different way.

By adding "CONFIG_" prefix, this macro appears
in include/autoconf.mk, include/spl-autoconf.mk.
(And treating USE_PRIVATE_LIBGCC as CONFIG macro
is reasonable enough.)

Tegra SoC family defined USE_PRIVATE_LIBGCC as "yes"
in arch/arm/cpu/arm720t/tegra*/config.mk,
whereas did not define it in arch/arm/cpu/armv7/tegra*/config.mk.

It means Tegra enables PRIVATE_LIBGCC only for SPL.
We can describe the same behavior by adding

  #ifdef CONFIG_SPL_BUILD
  # define CONFIG_USE_PRIVATE_LIBGCC
  #endif

to include/configs/tegra-common.h.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Warren <twarren@nvidia.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>