x86: select CONFIG_64BIT for X86_64

Select CONFIG_64BIT so that we pass the -m64 option (instead of -m32) to
static analysis tools.
Introduce CONFIG_SPL_64BIT and select it for architectures other than
x86 with 64 bit builds. Do not select it for x86 builds as x86 uses
a 32 bit SPL.
Ensure that when limits are set they use CONFIG_64BIT for U-Boot
proper and CONFIG_SPL_64BIT for SPL. This is to allow for the 32 bit
SPL build used by x86.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
diff --git a/arch/Kconfig b/arch/Kconfig
index 6258788..bb2e7be 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -37,6 +37,14 @@
 
 config 64BIT
 	bool
+	help
+	  Indicates that U-Boot proper will be built for a 64 bit
+	  architecture.
+
+config SPL_64BIT
+	bool
+	help
+	  Indicates that SPL will be built for a 64 bit architecture.
 
 config SYS_CACHELINE_SIZE
 	int