LibC: Implement some mem* and str* functions in assembly

This made them a lot faster on modern cpus with optimized rep stos and
rep movs

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

@@ -0,0 +1,62 @@
+.global memchr
+memchr:
+	movb %sil, %al
+	movq %rdx, %rcx
+	repne scasb
+	xorq %rax, %rax
+	testq %rcx, %rcx
+	cmovnzq %rdi, %rax
+	ret
+
+.global memcmp
+memcmp:
+	movq %rdx, %rcx
+	repe cmpsb
+	jne .memcmp_not_equal
+	xorq %rax, %rax
+	ret
+ .memcmp_not_equal:
+	movzbl -1(%rdi), %eax
+	movzbl -1(%rsi), %ecx
+	subq %rcx, %rax
+	ret
+
+.global memcpy
+memcpy:
+	movq %rdi, %rax
+	movq %rdx, %rcx
+	movq %rdi, %rdx
+	rep movsb
+	movq %rdx, %rax
+	ret
+
+.global memmove
+memmove:
+	cmpq %rdi, %rsi
+	jae memcpy
+	leaq -1(%rdi, %rdx), %rdi
+	leaq -1(%rsi, %rdx), %rsi
+	movq %rdx, %rcx
+	std
+	rep movsb
+	cld
+	leaq 1(%rdi), %rax
+	ret
+
+.global memset
+memset:
+	movq %rdi, %r8
+	movb %sil, %al
+	movq %rdx, %rcx
+	rep stosb
+	movq %r8, %rax
+	ret
+
+.global strlen
+strlen:
+	xorb %al, %al
+	movq $-1, %rcx
+	repne scasb
+	movq $-2, %rax
+	subq %rcx, %rax
+	ret