[build] fix build def to use current_cpu

This change fixes the build config to use current_cpu instead of
target_cpu. The former propagates toolchain overrides, while the
latter does not.

Test: built core.arm64 and core.x64
Change-Id: Id5bcad90a612a63756df7fa1e1ade9308fe461c1
diff --git a/BUILD.gn b/BUILD.gn
index f75eb43..16e173a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -35,13 +35,13 @@
 config("ffi_headers") {
   visibility = [ ":*" ]
   include_dirs = [ "./include" ]
-  if (target_cpu == "x64") {
+  if (current_cpu == "x64") {
     include_dirs += [
       "./include/x86",
       "${target_gen_dir}/x86",
     ]
   }
-  if (target_cpu == "arm64") {
+  if (current_cpu == "arm64") {
     include_dirs += [
       "./include/arm",
       "${target_gen_dir}/arm",
@@ -64,7 +64,7 @@
     "src/types.c",
   ]
 
-  if (target_cpu == "x64") {
+  if (current_cpu == "x64") {
     include_dirs += [ "./src/x86" ]
     deps = [
       ":target_x86",
@@ -77,7 +77,7 @@
     ]
   }
 
-  if (target_cpu == "arm64") {
+  if (current_cpu == "arm64") {
     include_dirs += [ "./src/arm" ]
     deps = [
       ":target_arm",