Summary:
Related to: https://github.com/facebook/rocksdb/pull/9215
* Adds build_detect_platform support for RISCV on Linux (at least on SiFive Unmatched platforms)
This still leaves some linking issues on RISCV remaining (e.g. when building `db_test`):
```
/usr/bin/ld: ./librocksdb_debug.a(memtable.o): in function `__gnu_cxx::new_allocator<char>::deallocate(char*, unsigned long)':
/usr/include/c++/10/ext/new_allocator.h:133: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: ./librocksdb_debug.a(memtable.o): in function `std::__atomic_base<bool>::compare_exchange_weak(bool&, bool, std::memory_order, std::memory_order)':
/usr/include/c++/10/bits/atomic_base.h:464: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: /usr/include/c++/10/bits/atomic_base.h:464: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: /usr/include/c++/10/bits/atomic_base.h:464: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: /usr/include/c++/10/bits/atomic_base.h:464: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: ./librocksdb_debug.a(memtable.o):/usr/include/c++/10/bits/atomic_base.h:464: more undefined references to `__atomic_compare_exchange_1' follow
/usr/bin/ld: ./librocksdb_debug.a(db_impl.o): in function `rocksdb::DBImpl::NewIteratorImpl(rocksdb::ReadOptions const&, rocksdb::ColumnFamilyData*, unsigned long, rocksdb::ReadCallback*, bool, bool)':
/home/adamretter/rocksdb/db/db_impl/db_impl.cc:3019: undefined reference to `__atomic_exchange_1'
/usr/bin/ld: ./librocksdb_debug.a(write_thread.o): in function `rocksdb::WriteThread::Writer::CreateMutex()':
/home/adamretter/rocksdb/./db/write_thread.h:205: undefined reference to `__atomic_compare_exchange_1'
/usr/bin/ld: ./librocksdb_debug.a(write_thread.o): in function `rocksdb::WriteThread::SetState(rocksdb::WriteThread::Writer*, unsigned char)':
/home/adamretter/rocksdb/db/write_thread.cc:222: undefined reference to `__atomic_compare_exchange_1'
collect2: error: ld returned 1 exit status
make: *** [Makefile:1449: db_test] Error 1
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9366
Reviewed By: jay-zhuang
Differential Revision: D34377664
Pulled By: mrambacher
fbshipit-source-id: c86f9d0cd1cb0c18de72b06f1bf5847f23f51118
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