fix(kernel,boot): improve GCC/Clang compatibility across kernel and boot components

[kheersagarpatel]

Summary

This PR addresses a set of GCC/Clang compatibility issues across the kernel, bootloader, and AArch64 build system while preserving the existing implementation where possible. The changes focus on resolving compiler-specific incompatibilities, correcting build configuration, and improving cross-toolchain portability.

Changes

Kernel

• Before: Some source files relied on compiler-specific types, contained invalid return statements, or included headers that caused portability issues with GCC/Clang.
• What: Added compiler and architecture guards where required, replaced compiler-specific variadic types with standard va_list, corrected non-void return paths, and removed an unnecessary host Linux header dependency.
• Why: Improve compatibility with GCC/Clang while preserving existing behavior.

Bootloader

• Before: The bootloader contained an incorrect MSVC compiler macro check and required updates for the GCC/Clang toolchain.
• What: Corrected the _MSC_VER preprocessor check and updated the AArch64 bootloader build configuration.
• Why: Improve compiler compatibility and maintain consistent build behavior across supported toolchains.

Build System

• Before: Default build target selection and AArch64 build configuration were not aligned with the intended GCC workflow.
• What: Updated the AArch64 Makefiles to use the intended default build target and adjusted linker/build configuration where required.
• Why: Improve build consistency for GCC/Clang-based development.

Compatibility

• Existing functionality is intended to remain unchanged.
• MSVC/Windows build verification pending.
• GCC/Linux build completed successfully.
• ARM64 runtime validated.

Related

Part of #44