Issue URL:
8282466: riscv: Remove unused code in linux_riscv #62
8282415: riscv: Rename StubRoutines::riscv64 to StubRoutines::riscv #60
8278895: riscv: Rename Riscv64 to RISCV64 #33
These three patches are doing rename work, so put them together.#33 Rename Riscv64 to RISCV64 for consistence. #60 Rename SubRoutine::riscv64 to SubRoutine::riscv, just like the PPC64 port. #62 More renaming from riscv64 to riscv in riscv backend, In jdk11, we need to rename more.

8278833: riscv: Remove the x3 and x4 register saving logic in register savers #31
x3 is the global pointer register used to reference global variables in C++ code, generated by linker; and x4 is the thread pointer register and would be modified by the kernel. we have no other modifications for the x3 & x4 registers so I would recommend removing the x3 & x4 saving logic in register savers.

8278474: riscv: Enable two more jtreg hotspot tests #I57U27
enable two more hotspot tests on risc(in jdk11 only one)

8278337: riscv: remove unnecessary ld/sd zr around calls #25
zr(x0) is used as stack alignment when saving/restoring registers around calls. But it's unnecessary to ld/st x0 from/to stack.