Add riscv64 arch and u74mc uarch (archspec#35)

Add riscv64 arch and SiFive u74mc uarch.

First version of GCC with RISC-V support was 7.1.
First version of LLVM with RISC-V support was 9.0.
rv64gc is a commonly used shorthand alias for rv64imafdc.

Author: ktpedre

cpu/microarchitectures.json

@@ -2017,6 +2017,44 @@
       "features": [],
       "compilers": {
       }
+    },
+    "riscv64": {
+      "from": [],
+      "vendor": "generic",
+      "features": [],
+      "compilers": {
+        "gcc": [
+          {
+            "versions": "7.1:",
+            "flags" : "-march=rv64gc"
+          }
+        ],
+        "clang": [
+          {
+            "versions": "9.0:",
+            "flags" : "-march=rv64gc"
+          }
+        ]
+      }
+    },
+    "u74mc": {
+      "from": ["riscv64"],
+      "vendor": "SiFive",
+      "features": [],
+      "compilers": {
+        "gcc": [
+          {
+            "versions": "10.2:",
+            "flags" : "-march=rv64gc -mtune=sifive-7-series"
+          }
+        ],
+        "clang" : [
+          {
+            "versions": "12.0:",
+            "flags" : "-march=rv64gc -mtune=sifive-7-series"
+          }
+        ]
+      }
     }
   },
   "feature_aliases": {

tests/targets/linux-sifive-u74mc

@@ -0,0 +1,5 @@
+processor       : 0
+hart            : 4
+isa             : rv64imafdc
+mmu             : sv39
+uarch           : sifive,u74-mc