Add target armv7s-darwin-gcc.

Really just armv7. This is a convenience target intended to make iOS
development with libvpx easier. Xcode projects with default settings
will fail to build when a framework lacks armv7s support when targetting
iOS7.

Change-Id: I7eb80d52eec25501febc0d2c3c0b4ed964b8ed5b

configure

diff --git a/configure b/configure
index 0cacc4c..800553e 100755
--- a/configure
+++ b/configure
@@ -103,6 +103,7 @@
 all_platforms="${all_platforms} armv7-none-rvct"     #neon Cortex-A8
 all_platforms="${all_platforms} armv7-win32-vs11"
 all_platforms="${all_platforms} armv7-win32-vs12"
+all_platforms="${all_platforms} armv7s-darwin-gcc"
 all_platforms="${all_platforms} mips32-linux-gcc"
 all_platforms="${all_platforms} ppc32-darwin8-gcc"
 all_platforms="${all_platforms} ppc32-darwin9-gcc"