ARM: improve armcc detection

Apparently some versions of armcc do not identify as RVCT.  This
changes the test to a string used by all versions.

Originally committed as revision 19985 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1536,7 +1536,7 @@ elif $cc -V 2>/dev/null | grep -q Compaq; then
     DEPFLAGS='$(CPPFLAGS) $(CFLAGS) -M'
     debuglevel=3
     add_ldflags -Wl,-z,now # calls to libots crash without this
-elif $cc --vsn 2>/dev/null | grep -q RVCT; then
+elif $cc --vsn 2>/dev/null | grep -q "ARM C/C++ Compiler"; then
     test -d "$sysroot" || die "No valid sysroot specified."
     cc_type=armcc
     cc_version="AV_STRINGIFY(__ARMCC_VERSION)"