[lldb] Add RISCV for Makefile.rules (llvm#124758)

As discussed with @DavidSpickett in discord. Running the test runner
caused the following issue:

```
gmake: Entering directory '/home/kper/oss/llvm-project/build/lldb-test-build.noindex/functionalities/thread/concurrent_events/TestConcurrentSignalWatch.test'
/home/kper/oss/llvm-project/build/bin/clang++  -std=c++11 -g -O0  -mriscv -I/home/kper/oss/llvm-project/lldb/packages/Python/lldbsuite/test/make/../../../../..//include -I/home/kper/oss/llvm-project/build/tools/lldb/include -I/home/kper/oss/llvm-project/lldb/test/API/functionalities/thread/concurrent_events -I/home/kper/oss/llvm-project/lldb/packages/Python/lldbsuite/test/make -include /home/kper/oss/llvm-project/lldb/packages/Python/lldbsuite/test/make/test_common.h -fno-limit-debug-info      --driver-mode=g++ -MT main.o -MD -MP -MF main.d -c -o main.o /home/kper/oss/llvm-project/lldb/test/API/functionalities/thread/concurrent_events/main.cpp
clang++: error: unknown argument: '-mriscv'
gmake: *** [Makefile.rules:619: main.o] Error 1
```

By overriding the flags, we avoid the `-mriscv`.

Author: kper

lldb/packages/Python/lldbsuite/test/make/Makefile.rules

@@ -207,6 +207,10 @@ else
 		override ARCH :=
 		override ARCHFLAG :=
 	endif
+	ifeq "$(ARCH)" "riscv"
+		override ARCH :=
+		override ARCHFLAG :=
+	endif
 	ifeq "$(findstring mips,$(ARCH))" "mips"
 		override ARCHFLAG := -
 	endif