Crypto: document that CRYPTOPP_ALIGN_DATA can expand to nothing

[bernardladenthin]

What

Adds a two-line comment to the CRYPTOPP_ALIGN_DATA definition in src/Crypto/config.h. No functional change.

Why

The macro has three branches, and the last one is empty:

#ifndef CRYPTOPP_ALIGN_DATA
    #if defined(_MSC_VER) && !defined(TC_WINDOWS_BOOT)
        #define CRYPTOPP_ALIGN_DATA(x) __declspec(align(x))
    #elif defined(__GNUC__)
        #define CRYPTOPP_ALIGN_DATA(x) __attribute__((aligned(x)))
    #else
        #define CRYPTOPP_ALIGN_DATA(x)      // <-- silently no alignment
    #endif
#endif

Under MSVC with TC_WINDOWS_BOOT the first branch is skipped, __GNUC__ is not defined either, so the macro expands to nothing. This is not visible at the ~79 call sites that use it.

The caveat is reachable, not hypothetical: src/Common/Xts.c contains no TC_WINDOWS_BOOT guards and is compiled into the bootloader, where its two CRYPTOPP_ALIGN_DATA(16) whiteningValue[] declarations lose their alignment. That is harmless there — bootloader code is real-mode without SSE — but it is not obvious from reading either file.

Scope

Documentation only. The behaviour is intentional and is deliberately left unchanged.