Whitelist v7 feature for ARM and AARCH64.

Needed for `v7` features in `coresimd`. 

See https://github.com/rust-lang-nursery/stdsimd/blob/b2f7be24d5043a88427f9a5258ca9a51ede6d029/coresimd/src/arm/v7.rs#L40 which used to work but doesn't anymore.

r? alexcrichton

src/librustc_trans/llvm_util.rs

@@ -79,9 +79,9 @@ unsafe fn configure_llvm(sess: &Session) {
 // detection code will walk past the end of the feature array,
 // leading to crashes.
 
-const ARM_WHITELIST: &'static [&'static str] = &["neon\0", "vfp2\0", "vfp3\0", "vfp4\0"];
+const ARM_WHITELIST: &'static [&'static str] = &["neon\0", "v7\0", "vfp2\0", "vfp3\0", "vfp4\0"];
 
-const AARCH64_WHITELIST: &'static [&'static str] = &["neon\0"];
+const AARCH64_WHITELIST: &'static [&'static str] = &["neon\0", "v7\0"];
 
 const X86_WHITELIST: &'static [&'static str] = &["avx\0", "avx2\0", "bmi\0", "bmi2\0", "sse\0",
                                                  "sse2\0", "sse3\0", "sse4.1\0", "sse4.2\0",