fate.sh: Allow specifying --ar through a separate variable

This avoids needing to use the extra_conf variable. That variable
is problematic for setting a value that contains spaces.

This adds options for another tool in the same fashion as other
tools were added in 523d688c2b.

doc/fate_config.sh.template

@@ -14,6 +14,7 @@ arch=
 cpu=
 toolchain=
 cross_prefix=
+ar=
 as=
 cc=
 cxx=

tests/fate.sh

@@ -53,6 +53,7 @@ configure()(
         ${cpu:+--cpu="$cpu"}                                            \
         ${toolchain:+--toolchain="$toolchain"}                          \
         ${cross_prefix:+--cross-prefix="$cross_prefix"}                 \
+        ${ar:+--ar="$ar"}                                               \
         ${as:+--as="$as"}                                               \
         ${cc:+--cc="$cc"}                                               \
         ${cxx:+--cxx="$cxx"}                                            \