configure: arm: detect toolchain default arch version

Probe for the toolchain default architecture version if no --cpu flag
is present or an unknown cpu is specified.  Works with gcc, clang and
armcc.

This allows configuring based on the arch version even if it is not
explicitly specified to configure.  It also causes an explicit -march
flag to be added to CFLAGS and ASFLAGS, which in turn lets us do
proper instruction set tests with the assembler.

Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -2541,6 +2541,37 @@ elif enabled alpha; then
 
 elif enabled arm; then
 
+    check_arm_arch() {
+        check_cpp_condition stddef.h \
+            "defined __ARM_ARCH_${1}__ || defined __TARGET_ARCH_${2:-$1}" \
+            $cpuflags
+    }
+
+    probe_arm_arch() {
+        if   check_arm_arch 4;        then echo armv4;
+        elif check_arm_arch 4T;       then echo armv4t;
+        elif check_arm_arch 5;        then echo armv5;
+        elif check_arm_arch 5E;       then echo armv5e;
+        elif check_arm_arch 5T;       then echo armv5t;
+        elif check_arm_arch 5TE;      then echo armv5te;
+        elif check_arm_arch 5TEJ;     then echo armv5te;
+        elif check_arm_arch 6;        then echo armv6;
+        elif check_arm_arch 6J;       then echo armv6j;
+        elif check_arm_arch 6K;       then echo armv6k;
+        elif check_arm_arch 6Z;       then echo armv6z;
+        elif check_arm_arch 6ZK;      then echo armv6zk;
+        elif check_arm_arch 6T2;      then echo armv6t2;
+        elif check_arm_arch 7;        then echo armv7;
+        elif check_arm_arch 7A  7_A;  then echo armv7-a;
+        elif check_arm_arch 7R  7_R;  then echo armv7-r;
+        elif check_arm_arch 7M  7_M;  then echo armv7-m;
+        elif check_arm_arch 7EM 7E_M; then echo armv7-m;
+        elif check_arm_arch 8A  8_A;  then echo armv8-a;
+        fi
+    }
+
+    [ "$cpu" = generic ] && cpu=$(probe_arm_arch)
+
     case $cpu in
         armv*)
             cpuflags="-march=$cpu"
@@ -2555,6 +2586,7 @@ elif enabled arm; then
                 arm11*)                                  subarch=armv6   ;;
                 arm[79]*e*|arm9[24]6*|arm96*|arm102[26]) subarch=armv5te ;;
                 armv4*|arm7*|arm9[24]*)                  subarch=armv4   ;;
+                *)                             subarch=$(probe_arm_arch) ;;
             esac
         ;;
     esac