configure: add fallback to $arch in msvc assembler check.

Setting the defaults for $arch happens only later, so
the current code would not set AS correctly if --arch
was not specified on the command-line.
Fix it by adding an explicit fallback to $arch_default.

Signed-off-by: Josh Dekker <josh@itanimul.li>

configure

@@ -4268,7 +4268,7 @@ case "$toolchain" in
         ld_default="$source_path/compat/windows/mslink"
         nm_default="dumpbin.exe -symbols"
         ar_default="lib.exe"
-        case "$arch" in
+        case "${arch:-$arch_default}" in
         aarch64|arm64)
             as_default="armasm64.exe"
             ;;