LibC: Fix memchr and memcmp assembly implementations

These functions did not work with size 0 :D

userspace/libraries/LibC/arch/i686/string.S

@@ -3,10 +3,11 @@ memchr:
 	xchgl 4(%esp), %edi
 	movl  8(%esp), %eax
 	movl 12(%esp), %ecx
+	movl $1, %edx
+	cmpl $1, %ecx # clear ZF if count is zero
 	repne scasb
-	xorl %eax, %eax
-	testl %ecx, %ecx
-	cmovnzl %edi, %eax
+	cmovel %edi, %edx
+	leal -1(%edx), %eax
 	movl 4(%esp), %edi
 	ret
 
@@ -15,6 +16,7 @@ memcmp:
 	xchgl 4(%esp), %edi
 	xchgl 8(%esp), %esi
 	movl 12(%esp), %ecx
+	testl %ecx, %ecx # set ZF if count is zero
 	repe cmpsb
 	jne .memcmp_not_equal
 	xorl %eax, %eax

userspace/libraries/LibC/arch/x86_64/string.S

@@ -2,15 +2,17 @@
 memchr:
 	movb %sil, %al
 	movq %rdx, %rcx
+	movq $1, %rdx
+	cmpq $1, %rcx # clear ZF if count is zero
 	repne scasb
-	xorq %rax, %rax
-	testq %rcx, %rcx
-	cmovnzq %rdi, %rax
+	cmoveq %rdi, %rdx
+	leaq -1(%rdx), %rax
 	ret
 
 .global memcmp
 memcmp:
 	movq %rdx, %rcx
+	testq %rcx, %rcx # set ZF if count is zero
 	repe cmpsb
 	jne .memcmp_not_equal
 	xorq %rax, %rax