Make ar invocation more portable

armar doesn't understand the syntax without dash. OTOH, the syntax with dash
is the only one specified by POSIX, and it's accepted by GNU ar, BSD ar (as
bundled with OS X) and armar, so it looks like the most portable syntax.

fixes #386
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 5ddb035..c3369ea 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -234,7 +234,7 @@
 scripts/config.pl unset MBEDTLS_THREADING_C
 scripts/config.pl unset MBEDTLS_MEMORY_BACKTRACE # execinfo.h
 scripts/config.pl unset MBEDTLS_MEMORY_BUFFER_ALLOC_C # calls exit
-CC=armcc WARNING_CFLAGS= make lib 2> armcc.stderr
+CC=armcc AR=armar WARNING_CFLAGS= make lib 2> armcc.stderr
 if [ -s armcc.stderr ]; then
     cat armcc.stderr
     exit 1;