RFR: 8317257: RISC-V: llvm build broken

[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.

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

Commit messages:
 - fix compilation

Changes: https://git.openjdk.org/jdk/pull/15965/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15965&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317257
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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