RFR: 8291947: riscv: fail to build after JDK-8290840

[Yadong Wang]

[JDK-8290840](https://bugs.openjdk.org/browse/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.

Tier1 of hotspot and jdk passed on unmatched.

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Commit messages:
 - 8291947: riscv: fail to build after JDK-8290840

Changes: https://git.openjdk.org/jdk/pull/9763/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9763&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291947
  Stats: 20 lines in 1 file changed: 0 ins; 19 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9763.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9763/head:pull/9763

PR: https://git.openjdk.org/jdk/pull/9763