[test-suite] Add Architecture Detection for RISC-V

[lenary]

Summary

The LLVM test suite has its own way of detecting the system
architecture. This adds support in that file for detecting RISC-V.

This will eventually cause the ARCH variable to be populated.

We use ARCH="riscv64" to identify 64-bit RISC-V, and ARCH="riscv32" to identify 32-bit
RISC-V, so that attempting to detect "riscv" in the ARCH variable will match any version of RISC-V.

[luismarques]

LGTM. I suggest slightly tweaking the commit message to make it clear that it's the str message that is prefix matched for riscv.

cmake/modules/DetectArchitecture.c

 #if defined(__aarch64__)
 const char *str = "ARCHITECTURE IS AArch64";
 #elif defined(__arm__)
 const char *str = "ARCHITECTURE IS ARM";
 #elif defined(__alpha__)
 const char *str = "ARCHITECTURE IS Alpha";
 #elif defined(__mips__)
 const char *str = "ARCHITECTURE IS Mips";
 #elif defined(__powerpc__) || defined(__ppc__) || defined(__power__)
 const char *str = "ARCHITECTURE IS PowerPC";
+#elif defined(__riscv)
+#if __riscv_xlen == 64
+const char *str = "ARCHITECTURE IS riscv64";
+#elif __riscv_xlen == 32
+const char *str = "ARCHITECTURE IS riscv32";
+#endif
 #elif defined(__s390__)
 const char *str = "ARCHITECTURE IS SystemZ";
 #elif defined(__sparc__)
 const char *str = "ARCHITECTURE IS Sparc";
 #elif defined(__xcore__)
 const char *str = "ARCHITECTURE IS XCore";
 #elif defined(__i386__) || defined(__x86_64__)
 const char *str = "ARCHITECTURE IS x86";
 #endif
 
 int main(int argc, char **argv) {
     int require = str[argc];
     (void)argv;
     return require;
 }