arch/x86: Add 64-bit U-Boot configuration without SPL

This commit introduces a new configuration option X86_RUN_64BIT_NO_SPL
to allow building U-Boot as a 64-bit binary without using the SPL
(Secondary Program Loader). The motivation is to simplify the boot
process for certain x86-based platforms that do not require SPL, such as
those booting directly from a 64-bit coreboot firmware.

This update revises the `X86_RUN_64BIT` configuration to more accurately
describe its role as "32-bit SPL followed by 64-bit U-Boot." It
clarifies the sequence of operations during the boot process, where the
system transitions from a 32-bit SPL (Secondary Program Loader) to the
main 64-bit U-Boot.

Signed-off-by: Jeremy Compostella <[email protected]>
Reviewed-by: Simon Glass <[email protected]>

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