libroot_x86: use the generic memset/memcpy when x86_64 is the primary arch.

the standard x86 implementation uses the commpage, which is difficult to setup. we could
also use the C++11 version instead.

Change-Id: I4be7e3466082ff2c91bc32bef377a664ce65f3fc

src/system/libroot/posix/string/arch/generic/memcpy.c

@@ -6,8 +6,6 @@
 #include <sys/types.h>
 #include <string.h>
 
-/* do not build for for arches that have overridden this with an assembly version */
-#if !defined(ARCH_x86)
 
 typedef int word;
 
@@ -45,5 +43,3 @@ void *memcpy(void *dest, const void *src, size_t count)
 	return dest;
 }
 
-#endif
-

src/system/libroot/posix/string/arch/x86/Jamfile

@@ -7,8 +7,12 @@ for architectureObject in [ MultiArchSubDirSetup x86 x86_gcc2 ] {
 
 		UsePrivateSystemHeaders ;
 
+		SEARCH_SOURCE += [ FDirName $(SUBDIR) $(DOTDOT) generic ] ;
+
 		MergeObject <$(architecture)>posix_string_arch_$(TARGET_ARCH).o :
-			arch_string.S
+			[ MultiArchIfPrimary memcpy.c memset.c :
+				arch_string.S : x86_64 ]
 			;
+
 	}
 }