Whitelist pclmul x86 feature flag

Relevant `stdsimd` [issue](rust-lang/stdarch#318).

src/librustc_trans/llvm_util.rs

@@ -88,7 +88,7 @@ const X86_WHITELIST: &'static [&'static str] = &["avx\0", "avx2\0", "bmi\0", "bm
                                                  "ssse3\0", "tbm\0", "lzcnt\0", "popcnt\0",
                                                  "sse4a\0", "rdrnd\0", "rdseed\0", "fma\0",
                                                  "xsave\0", "xsaveopt\0", "xsavec\0",
-                                                 "xsaves\0", "aes\0",
+                                                 "xsaves\0", "aes\0", "pclmul\0"
                                                  "avx512bw\0", "avx512cd\0",
                                                  "avx512dq\0", "avx512er\0",
                                                  "avx512f\0", "avx512ifma\0",