third_party: Add support for building libffi for ARM.

third_party/libffi/include/arm-unknown-linux-gnu/include/ffi.h

Index: third_party/libffi/include/arm-unknown-linux-gnu/include/ffi.h
diff --git a/third_party/libffi/include/x86_64-unknown-linux-gnu/include/ffi.h b/third_party/libffi/include/arm-unknown-linux-gnu/include/ffi.h
similarity index 99%
copy from third_party/libffi/include/x86_64-unknown-linux-gnu/include/ffi.h
copy to third_party/libffi/include/arm-unknown-linux-gnu/include/ffi.h
index 6b3a082c31406cf802c0ab8ac78541a7bf60c50b..b11d96bc0a0408b5577e30ec3f10577f82f3468b 100644
--- a/third_party/libffi/include/x86_64-unknown-linux-gnu/include/ffi.h
+++ b/third_party/libffi/include/arm-unknown-linux-gnu/include/ffi.h
@@ -58,8 +58,8 @@ extern "C" {
 #endif
 
 /* Specify which architecture libffi is configured for. */
-#ifndef X86_64
-#define X86_64
+#ifndef ARM
+#define ARM
 #endif
 
 /* ---- System configuration information --------------------------------- */
@@ -194,7 +194,7 @@ FFI_EXTERN ffi_type ffi_type_float;
 FFI_EXTERN ffi_type ffi_type_double;
 FFI_EXTERN ffi_type ffi_type_pointer;
 
-#if 1
+#if 0
 FFI_EXTERN ffi_type ffi_type_longdouble;
 #else
 #define ffi_type_longdouble ffi_type_double
@@ -203,7 +203,7 @@ FFI_EXTERN ffi_type ffi_type_longdouble;
 #ifdef FFI_TARGET_HAS_COMPLEX_TYPE
 FFI_EXTERN ffi_type ffi_type_complex_float;
 FFI_EXTERN ffi_type ffi_type_complex_double;
-#if 1
+#if 0
 FFI_EXTERN ffi_type ffi_type_complex_longdouble;
 #else
 #define ffi_type_complex_longdouble ffi_type_complex_double
@@ -460,7 +460,7 @@ void ffi_call(ffi_cif *cif,
 #define FFI_TYPE_INT        1
 #define FFI_TYPE_FLOAT      2    
 #define FFI_TYPE_DOUBLE     3
-#if 1
+#if 0
 #define FFI_TYPE_LONGDOUBLE 4
 #else
 #define FFI_TYPE_LONGDOUBLE FFI_TYPE_DOUBLE