embree: Fix Linux ARM64 build with `-flax-vector-conversions`

This is a change done upstream in the `devel3` branch for 3.13.6:
https://github.com/embree/embree/commit/82ca6b5ccb7abe0403a658a0e079926478f04cb1

They also seem to define it for macOS, but for us it breaks the build...
¯\_(ツ)_/¯

Also change wrong use of CPPFLAGS (pre-processor) where CXXFLAGS (C++)
makes more sense.

(cherry picked from commit b5a8055b5cf69a63e32045f433bf1e7fd39d49ad)

modules/raycast/SCsub

@@ -68,10 +68,10 @@ if env["builtin_embree"]:
 
     if not env.msvc:
         if env["arch"] in ["x86", "x86_64"]:
-            env_raycast.Append(CPPFLAGS=["-msse2", "-mxsave"])
+            env_raycast.Append(CCFLAGS=["-msse2", "-mxsave"])
 
         if env["platform"] == "windows":
-            env_raycast.Append(CPPFLAGS=["-mstackrealign"])
+            env_raycast.Append(CCFLAGS=["-mstackrealign"])
 
     if env["platform"] == "windows":
         if env.msvc:
@@ -90,8 +90,11 @@ if env["builtin_embree"]:
 
     if not env.msvc:
         # Flags synced with upstream gnu.cmake.
+        if env["arch"] == "arm64" and env["platform"] == "x11":
+            env_thirdparty.Append(CXXFLAGS=["-flax-vector-conversions"])
+
         env_thirdparty.Append(
-            CPPFLAGS=[
+            CXXFLAGS=[
                 "-fno-strict-overflow",
                 "-fno-delete-null-pointer-checks",
                 "-fwrapv",