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

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.

Reviewers: asb, luismarques

Reviewed By: luismarques

Subscribers: mgorny, simoncook, kito-cheng, shiva0217, rogfer01, rkruppe, PkmX, s.egerton, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68698

git-svn-id: https://llvm.org/svn/llvm-project/test-suite/trunk@374176 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/cmake/modules/DetectArchitecture.c b/cmake/modules/DetectArchitecture.c
index ef488b5..35f7c0b 100644
--- a/cmake/modules/DetectArchitecture.c
+++ b/cmake/modules/DetectArchitecture.c
@@ -8,6 +8,12 @@
 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";
 #elif defined(__s390__)
 const char *str = "ARCHITECTURE IS SystemZ";
 #elif defined(__sparc__)