Minimal RISC-V support

Sufficient to make CMake build for RISC-V successfully configure and build, and run microkernel-level tests

PiperOrigin-RevId: 372524850
diff --git a/CMakeLists.txt b/CMakeLists.txt
index ea38f50..8f29a61 100755
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -76,7 +76,7 @@
   ELSE()
     MESSAGE(FATAL_ERROR "CMAKE_SYSTEM_PROCESSOR is not defined")
   ENDIF()
-ELSEIF(NOT XNNPACK_TARGET_PROCESSOR MATCHES "^(i[3-6]86|x86|x86_64|AMD64|armv[5-8].*|aarch64|arm64.*)$")
+ELSEIF(NOT XNNPACK_TARGET_PROCESSOR MATCHES "^(i[3-6]86|x86|x86_64|AMD64|armv[5-8].*|aarch64|arm64.*|riscv(32|64|128))$")
   MESSAGE(FATAL_ERROR "Unrecognized XNNPACK_TARGET_PROCESSOR = ${XNNPACK_TARGET_PROCESSOR}")
 ENDIF()