Define Float16 clamp

Bug: http://b/27366867

clamp is defined in two files: driver/runtime/arch/generic.c (used to
build libclcore for most architectures) and driver/runtime/arch/clamp.c
(used to build libclcore for AArch64, ARM32 with Neon).  I had added
Float16 clamp in clamp.c but not in generic.c.  This CL adds the
functions to arch/generic.c.

Change-Id: I26df4f1efec6e909c786baf132d6c296b5a3468f
(cherry picked from commit 2173a9968276b2b91b7ed3d0a76d9e642710843b)
diff --git a/driver/runtime/arch/generic.c b/driver/runtime/arch/generic.c
index 92e7b8b..b5e5971 100644
--- a/driver/runtime/arch/generic.c
+++ b/driver/runtime/arch/generic.c
@@ -98,6 +98,7 @@
 
 #endif // !defined(ARCH_X86_HAVE_SSSE3) || defined(RS_DEBUG_RUNTIME) || defined(RS_G_RUNTIME)
 
+_CLAMP(half);
 _CLAMP(double);
 _CLAMP(char);
 _CLAMP(uchar);