configure: arm: Don't add -march= to the compiler if no preference was passed

If no --cpu= option was passed to configure, we detect what the
compiler defaults to. This detected value was then fed back to the
rest of the configure logic, as if it was an explicit choice.

This breaks on Ubuntu 21.10 with GCC 11.1.

Since GCC 8, it's possible to add configure extra features via the
-march option, like e.g. -march=armv7-a+neon. If the -mfpu= option
is configured to default to 'auto', the fpu setting gets taken
from the -march option.

GCC 11.1 in Ubuntu seems to be configured to use -mfpu=auto. This
has the effect of breaking any compilation command that specifies
-march=armv7-a, because the driver implicitly also adds -mfloat-abi=hard,
and that combination results in this error:

    cc1: error: ‘-mfloat-abi=hard’: selected processor lacks an FPU

One can compile successfully by passing e.g. -march=armv7-a+fp.

Therefore, restructure configure. If no specific preference was set
(and the 'cpu' configure variable was set as the output of
probe_arm_arch), the value we tried to set via -march= was the same
value that we just tried to detect as the compiler default.

So instead, just try to detect what the compiler defaults to, with
to allow setting other configure settings (such as 'fast_unaligned'),
but don't try to spell out the compiler's default via the -march flag.

Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -5009,9 +5009,11 @@ elif enabled arm; then
         fi
     }
 
-    [ "$cpu" = generic ] && cpu=$(probe_arm_arch)
 
     case $cpu in
+        generic)
+            subarch=$(probe_arm_arch | sed 's/[^a-z0-9]//g')
+        ;;
         armv*)
             cpuflags="-march=$cpu"
             subarch=$(echo $cpu | sed 's/[^a-z0-9]//g')