RFR: 8317257: RISC-V: llvm build broken [v2]

[Vladimir Kempik]

> Please review this small fix to make hotspot compilable with clang on risc-v.
> It supposed to fix next error:
> 
> /home/user/openjdk/jdk/src/hotspot/cpu/riscv/c1_LIRAssembler_riscv.cpp:1518:10: error: call to member function 'null_check' is ambiguous
>       __ null_check(obj);
>       ~~~^~~~~~~~~~
> /home/user/openjdk/jdk/src/hotspot/cpu/riscv/macroAssembler_riscv.hpp:238:16: note: candidate function
>   virtual void null_check(Register reg, int offset = -1);
>                ^
> /home/user/openjdk/jdk/src/hotspot/cpu/riscv/c1_MacroAssembler_riscv.hpp:109:8: note: candidate function
>   void null_check(Register r, Label *Lnull = nullptr) { MacroAssembler::null_check(r); }
>        ^
> 1 error generated.

Vladimir Kempik has updated the pull request incrementally with one additional commit since the last revision:

  align with arm64 and static_assert should have a message if less than c++17

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15965/files
  - new: https://git.openjdk.org/jdk/pull/15965/files/f946defb..9743e5ca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15965&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15965&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15965.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15965/head:pull/15965

PR: https://git.openjdk.org/jdk/pull/15965