x86/boot: Copy kstrtoull() to boot/string.c

Copy kstrtoull() and the other necessary functions from lib/kstrtox.c
to boot/string.c so that code in boot/ can use kstrtoull() and the old
simple_strtoull() can gradually be phased out.

Using div_u64() from math64.h directly will cause the dividend to be
handled as a 64-bit value and cause the infamous __divdi3 linker error
due to gcc trying to use its library function for the 64-bit division.

Therefore, separate the dividend into an upper and lower part.

 [ bp: Rewrite commit message. ]

Signed-off-by: Chao Fan <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: "H. Peter Anvin" <[email protected]>
Cc: [email protected]
Cc: Ingo Molnar <[email protected]>
Cc: [email protected]
Cc: Kees Cook <[email protected]>
Cc: [email protected]
Cc: Thomas Gleixner <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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