Build libnettle with assembly code (#1292)

* Build libnettle with assembler code

* Add .gitattributes for shell script

* Patch nettle for CPU feature detection on Android

* Don't build libnettle at configuration time

* Fix build

* Refine

app/src/main/cpp/nettle.patch

@@ -0,0 +1,21 @@
+diff --git a/fat-arm64.c b/fat-arm64.c
+index aec99f66..62b3adea 100644
+--- a/fat-arm64.c
++++ b/fat-arm64.c
+@@ -43,9 +43,14 @@
+ #if defined(__linux__) && defined(__GLIBC__) && defined(__GLIBC_PREREQ)
+ # if __GLIBC_PREREQ(2, 16)
+ #  define USE_GETAUXVAL 1
+-#  include <asm/hwcap.h>
+-#  include <sys/auxv.h>
+ # endif
++#elif __ANDROID_API__ >= 18
++# define USE_GETAUXVAL 1
++#endif
++
++#if USE_GETAUXVAL
++# include <asm/hwcap.h>
++# include <sys/auxv.h>
+ #elif defined(__OpenBSD__)
+ # include <sys/sysctl.h>
+ # include <machine/cpu.h>