soc/integration/export: Rewrite and fix zicsr RISC-V workaround.

- Improve regex to be more robust (Was not working with binutils from Sifive).
- Improve readability.
- Only do it for RISC-V.
- Add comments.

litex/soc/integration/export.py

@@ -75,7 +75,7 @@ def get_cpu_mak(cpu, compile_software):
         for i in range(len(triple)):
             t = triple[i]
             # Use native toolchain if host and target platforms are the same.
-            if t == 'riscv64-unknown-elf' and p == 'linux-riscv64':
+            if t == "riscv64-unknown-elf" and p == "linux-riscv64":
                 r = '--native--'
                 break
             if which(t+"-gcc"):
@@ -89,12 +89,25 @@ def get_cpu_mak(cpu, compile_software):
                 msg += "- " + triple[i] + "\n"
             raise OSError(msg)
         return r
-
     selected_triple = select_triple(triple)
-    if not clang:
-      binutils_version = re.match("GNU ar \(GNU Binutils\) (.+)", os.popen(selected_triple + "-ar -V").read()).group(1).split(".")
-      if int(binutils_version[1]) >= 2 and int(binutils_version[2]) >= 37 and (not re.search("zicsr", flags)):
-        flags = re.compile("-march=([^ ]+)").sub("-march=\\1_zicsr", flags)
+
+    # RISC-V's march zicsr workaround (for binutils >= 2.37).
+    def get_binutils_version():
+        version = 0
+        for i, l in enumerate(os.popen(selected_triple + "-ar -V")):
+            # Version is last float reported in first line.
+            if i == 0:
+                version = float(re.findall("\d+\.\d+", l)[-1])
+        return version
+
+    def apply_riscv_zicsr_march_workaround(flags):
+        # Append _zicsr to march when binutils >= 2.37 and zicsr is not present.
+        if (get_binutils_version() >= 2.37) and ("zicsr" not in flags):
+            flags = re.compile("-march=([^ ]+)").sub("-march=\\1_zicsr", flags)
+        return flags
+
+    if (not clang) and ("riscv" in selected_triple):
+        flags = apply_riscv_zicsr_march_workaround(flags)
 
     # Return informations.
     return [