Disable header-hygiene warnings on all platforms

Previously -Wno-header-hygiene was added to cflags only on x64, so the
compiler was generating a lot of warnings when compiling for arm64.
Now this flag is added on all platforms.

Also removed -Wno-null-dereference - it doesn't generate any warnings.

Change-Id: Id145404dba169da64a6febdec2732aec681a11bd
Reviewed-on: https://swiftshader-review.googlesource.com/c/22028
Tested-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>

third_party/llvm-7.0/BUILD.gn

diff --git a/third_party/llvm-7.0/BUILD.gn b/third_party/llvm-7.0/BUILD.gn
index 5876c3c..ec2fd43 100644
--- a/third_party/llvm-7.0/BUILD.gn
+++ b/third_party/llvm-7.0/BUILD.gn
@@ -54,15 +54,13 @@
     cflags += [ "-Wno-error" ]
   }
 
-  if (is_clang && (target_cpu == "x86" || target_cpu == "x64")) {
-    cflags += [
-      "-msse2",
-      "-Wno-header-hygiene",
-      "-Wno-null-dereference",
-    ]
-  } else {
-   # cflags += [ "-Wno-unused-but-set-variable" ]
+  if (is_clang) {
+    cflags += [ "-Wno-header-hygiene" ]
+    if (target_cpu == "x86" || target_cpu == "x64") {
+      cflags += [ "-msse2" ]
+    }
   }
+
   cflags += [
     "-Wno-attributes",
     "-Wno-deprecated-declarations",