common: compile shared lib, and export some c functions

[KenForever1]

Dear llama.cpp Official,

Hi, I'm writing to address this PR submission for allowing common lib to be compiled into both dynamic and static libraries, and export some c functions. In this way, we can load libcommon.so and call these functions with FFI call. For example, llama-cpp-python proj can call cpu_get_num_math function.

• I have read the contributing guidelines
• Self-reported review complexity:
  • Low
  • Medium
  • High

[bandoti]

@KenForever1 Although there are some useful routines in the common library, it is actually an implementation detail and not intended as a public interface. As such, if there are a couple of those routines needed (like the two in this PR: cpu_get_num_physical_cores and cpu_get_num_math), I recommend copying the logic and creating a custom shared library for your own needs.