riscv: fail to build after JDK-8290840

JDK-8290840 moved icache_flush from os_linux_riscv.cpp to icache_riscv.cpp, and we got 2 icache_flush there, and then caused a compilation error of "overloaded function with no contextual type information" in generate_icache_flush, which simply used the function name as a function address.

We put icache_flush in os files because it was just a linux implementation, and we saw some test failures on Hifive Unleashed when using the "portable" version of __builtin__clear_cache provided by compilers about 2 years ago. The problem doesn't exist anymore, so we can revert to the previous implementation of just using the builtin clear cache function.