Add option -fPIC (Position Independent Code) needed for x86_64.

While the generic atomic implementation is not used by default
for x86_64 or x86 builds, we still need to always build it with
-fPIC if we ever want to use it on these platforms.

lib/libspl/asm-generic/Makefile.am

@@ -7,7 +7,7 @@ atomic_SOURCE = atomic.c
 atomic_ASM = atomic.S
 
 COMPILE = $(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) \
-	  $(AM_CPPFLAGS) $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS)
+	  $(AM_CPPFLAGS) $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS) -fPIC
 EXTRA_DIST = ${atomic_SOURCE}
 
 # Generates assembly to simplify inclusion in ../Makefile.am