sys.h: Fix __attribute__ usage for non-GCC compilers

[mkannwischer]

Restructure MLD_INLINE and MLD_ALWAYS_INLINE into separate blocks, each independently guarded:

MLD_INLINE:

• MSVC: __inline
• C99 or inline macro defined: inline
• GCC/Clang C90: __attribute__((unused)) to silence warnings
• Other C90: empty

MLD_ALWAYS_INLINE:

• MSVC: __forceinline
• GCC/Clang C99+: MLD_INLINE __attribute__((always_inline))
• Other: MLD_INLINE (no forced inlining)

This fixes two issues:

1. If inline was defined as a macro before including sys.h, the MSVC check was bypassed and __attribute__ was used, causing MSVC failures.
2. For non-MSVC/GCC/Clang compilers like IAR, __attribute__ was used unconditionally, causing build failures.

Also fix typo: defined(clang) -> defined(__clang__)

• Ported from sys.h: Fix __attribute__ usage for non-GCC compilers mldsa-native#884

[mkannwischer]

The commit message still says MLD_... that needs to be changed to MLK_ everywhere.