[riscv-port-jdk17u:riscv-port] RFR: 8297644: RISC-V: Compilation error when shenandoah is disabled

[Gui Cao]

Please help review this backport to riscv-port-jdk17u.
Backport of [JDK-8297644](https://bugs.openjdk.org/browse/JDK-8297644). Applies cleanly.
The compilation failed before the modification as follows:

Creating support/modules_libs/java.base/server/libjvm.so from 975 file(s)
/home/zifeihan/riscv-port-jdk17u/src/hotspot/cpu/riscv/riscv.ad: In member function 'virtual void MachPrologNode::emit(CodeBuffer&, PhaseRegAlloc*) const':
/home/zifeihan/riscv-port-jdk17u/src/hotspot/cpu/riscv/riscv.ad:1351:7: error: invalid use of incomplete type 'class BarrierSetAssembler'
 1351 |     bs->nmethod_entry_barrier(&_masm);
      |       ^~
In file included from /home/zifeihan/riscv-port-jdk17u/src/hotspot/share/gc/shared/modRefBarrierSet.hpp:28,
                 from /home/zifeihan/riscv-port-jdk17u/src/hotspot/share/gc/shared/cardTableBarrierSet.hpp:29,
                 from /home/zifeihan/riscv-port-jdk17u/src/hotspot/cpu/riscv/riscv.ad:960,
                 from ad_riscv.cpp:31:
/home/zifeihan/riscv-port-jdk17u/src/hotspot/share/gc/shared/barrierSet.hpp:36:7: note: forward declaration of 'class BarrierSetAssembler'
   36 | class BarrierSetAssembler;
      |       ^~~~~~~~~~~~~~~~~~~
...
=== End of repeated output ===

No indication of failed target found.
Hint: Try searching the build log for '] Error'.
Hint: See doc/building.html#troubleshooting for assistance.

make[1]: *** [/home/zifeihan/riscv-port-jdk17u/make/Init.gmk:315: main] Error 2
make: *** [/home/zifeihan/riscv-port-jdk17u/make/Init.gmk:186: default] Error 2

Compile normally after fix.

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

Commit messages:
 - backport 8297644: RISC-V: Compilation error when shenandoah is disabled

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/22/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=22&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297644
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/22.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/22/head:pull/22

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/22