x86: select CONFIG_64BIT for X86_64

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>