Add __NDK_FPABI__ to function taking/returning float/double

Allow user code to be compiled with "-mhard-float" for armeabi-v7a
(which implies -mfloat-abi=hard), and either link with
1. Android native APIs which use softfp, or
2. A customized libm_hard.a which is compiled with -mhard-float. Need
   -D_NDK_MATH_NO_SOFTFP=1

See tests/device/hard-float/jni/Android.mk for details and restriction
on Clang

Change-Id: I773a842c73368e08b9a6cda0441e95a96fa303b2

ndk/platforms/android-9/include/sys/cdefs.h

@@ -499,4 +499,31 @@
 #define  __BIONIC__   1
 #include <android/api-level.h>
 
+/* __NDK_FPABI__ or __NDK_FPABI_MATH__ are applied to APIs taking or returning float or
+   [long] double, to ensure even at the presence of -mhard-float (which implies
+   -mfloat-abi=hard), calling to 32-bit Android native APIs still follow -mfloat-abi=softfp.
+
+   __NDK_FPABI_MATH__ is applied to APIs in math.h.  It normally equals to __NDK_FPABI__,
+   but allows use of customized libm.a compiled with -mhard-float by -D_NDK_MATH_NO_SOFTFP=1
+
+   NOTE: Disable for clang for now unless _NDK_MATH_NO_SOFTFP=1, because clang before 3.4 doesn't
+         allow change of calling convension for builtin and produces error message reads:
+
+           a.i:564:6: error: function declared 'aapcs' here was previously declared without calling convention
+           int  sin(double d) __attribute__((pcs("aapcs")));
+                ^
+           a.i:564:6: note: previous declaration is here
+ */
+#if defined(__ANDROID__) && !__LP64__ && defined( __arm__)
+#define __NDK_FPABI__ __attribute__((pcs("aapcs")))
+#else
+#define __NDK_FPABI__
+#endif
+
+#if (!defined(_NDK_MATH_NO_SOFTFP) || _NDK_MATH_NO_SOFTFP != 1) && !defined(__clang__)
+#define __NDK_FPABI_MATH__ __NDK_FPABI__
+#else
+#define __NDK_FPABI_MATH__  /* nothing */
+#endif
+
 #endif /* !_SYS_CDEFS_H_ */