build: Switch to static linking and adopt CROSS_COMPILE for toolchain selection

[agicy]

Problem Description

Currently, hvisor-tool defaults to dynamic linking. This causes issues when the build host's environment differs from the target machine's root-linux environment, leading to runtime errors such as missing shared libraries or version mismatches (e.g., glibc).

I encountered this issue when cross-compiling on an Ubuntu 22.04 (x86-64) host. The resulting binary failed to run on a Sysoul X3300 (Debian 11 aarch64) target.

Proposed Changes

The tool should be compiled statically to ensure portability across different rootfs versions and distributions.

The current build system relies on complex conditional logic to select the compiler. I suggest simplifying this by adopting the standard `CROSS_COMPILE` variable approach (similar to the Linux kernel build system). This would allow users to explicitly define their toolchain prefix easily.