x86/purgatory: Do not use fortified string functions

With the addition of -fstrict-flex-arrays=3, struct sha256_state's
trailing array is no longer ignored by CONFIG_FORTIFY_SOURCE:

struct sha256_state {
        u32 state[SHA256_DIGEST_SIZE / 4];
        u64 count;
        u8 buf[SHA256_BLOCK_SIZE];
};

This means that the memcpy() calls with "buf" as a destination in
sha256.c's code will attempt to perform run-time bounds checking, which
could lead to calling missing functions, specifically a potential
WARN_ONCE, which isn't callable from purgatory.

Reported-by: Thorsten Leemhuis <[email protected]>
Closes: https://lore.kernel.org/lkml/[email protected]/
Bisected-by: "Joan Bruguera Micó" <[email protected]>
Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: [email protected]
Cc: "H. Peter Anvin" <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: "Peter Zijlstra (Intel)" <[email protected]>
Cc: Alyssa Ross <[email protected]>
Cc: Sami Tolvanen <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Tested-by: Thorsten Leemhuis <[email protected]>
Acked-by: Dave Hansen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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