Add a patch for detection of AVX512 on Skylake

Author: tkelman

deps/llvm.mk

@@ -504,6 +504,7 @@ $(eval $(call LLVM_PATCH,llvm-D27389)) # Julia issue #19792, Remove for 4.0
 $(eval $(call LLVM_PATCH,llvm-D27397)) # Julia issue #19792, Remove for 4.0
 $(eval $(call LLVM_PATCH,llvm-D28009)) # Julia issue #19792, Remove for 4.0
 $(eval $(call LLVM_PATCH,llvm-D28215_FreeBSD_shlib))
+$(eval $(call LLVM_PATCH,llvm-D28221-avx512)) # mentioned in issue #19797
 endif # LLVM_VER
 
 ifeq ($(LLVM_VER),3.7.1)

deps/patches/llvm-D28221-avx512.patch

@@ -0,0 +1,22 @@
+Index: lib/Support/Host.cpp
+===================================================================
+--- a/lib/Support/Host.cpp
++++ b/lib/Support/Host.cpp
+@@ -475,13 +475,14 @@
+ 
+     // Skylake:
+     case 0x4e:
+-      *Type = INTEL_COREI7; // "skylake-avx512"
+-      *Subtype = INTEL_COREI7_SKYLAKE_AVX512;
+-      break;
+     case 0x5e:
+       *Type = INTEL_COREI7; // "skylake"
+       *Subtype = INTEL_COREI7_SKYLAKE;
+       break;
++    case 0x55:
++      *Type = INTEL_COREI7; // "skylake-avx512"
++      *Subtype = INTEL_COREI7_SKYLAKE_AVX512;
++      break;
+ 
+     case 0x1c: // Most 45 nm Intel Atom processors
+     case 0x26: // 45 nm Atom Lincroft