[libFuzzer] Refactor GetNextInstructionPc/GetPreviousInstructionPc

Port the change to compiler-rt/lib/fuzzer/FuzzerTracePC.cpp .
Update RISCV to use PC-2: this is coarse (C extension may be disabled) but
sufficient for pure symbolization purpose.

The commit is separate from D120362 so that bisecting/reverting is easier.