plat/common: move arch-agnostic fallback functions to C file

When we add a new callback, we need to duplicate fallbacks among
plat/common/{aarch32,aarch64}/platform_helpers.S  This is tedious.

I created a new C file, then moved 3 functions:
  plat_error_handler
  bl2_plat_preload_setup
  plat_try_next_boot_source

They are called from C, so I do not see a good reason to implement
them in assembly.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Makefile b/Makefile
index e32ba19..fab310b 100644
--- a/Makefile
+++ b/Makefile
@@ -193,6 +193,7 @@
 				lib/${ARCH}/cache_helpers.S		\
 				lib/${ARCH}/misc_helpers.S		\
 				plat/common/plat_log_common.c		\
+				plat/common/platform_helpers_default.c	\
 				plat/common/${ARCH}/plat_common.c	\
 				plat/common/${ARCH}/platform_helpers.S	\
 				${COMPILER_RT_SRCS}			\