fate.sh: Allow specifying --as via a specific variable

This simplifies specifying a value containing spaces for this
parameter.
Signed-off-by: Martin Storsjö <martin@martin.st>

doc/fate.texi

@@ -126,6 +126,7 @@ build_only=     # set to "yes" for a compile-only instance that skips tests
 arch=
 cpu=
 cross_prefix=
+as=
 cc=
 target_os=
 sysroot=

tests/fate.sh

@@ -48,6 +48,7 @@ configure()(
         ${arch:+--arch=$arch}                                           \
         ${cpu:+--cpu="$cpu"}                                            \
         ${cross_prefix:+--cross-prefix="$cross_prefix"}                 \
+        ${as:+--as="$as"}                                               \
         ${cc:+--cc="$cc"}                                               \
         ${ld:+--ld="$ld"}                                               \
         ${target_os:+--target-os="$target_os"}                          \