Revert "amd64: implement strlen in assembly"

This reverts commit af366d353b.

Trips over '\xa4' byte and terminates early, as found in
lib/libc/gen/setdomainname_test:setdomainname_basic testcase

However, keep moving libkern/strlen.c out of conf/files.

Reported by:	lwhsu

sys/amd64/amd64/support.S

@@ -697,72 +697,6 @@ ENTRY(fillw)
 	ret
 END(fillw)
 
-/*
- * strlen(string)
- *	  %rdi
- *
- * Uses the ((x - 0x01....01) & ~x & 0x80....80) trick.
- *
- * 0x01....01 is replaced with 0x0 - 0x01....01 so that it can be added
- * with leaq.
- *
- * For a description see either:
- * - "Hacker's Delight" by Henry S. Warren, Jr.
- * - "Optimizing subroutines in assembly language: An optimization guide for x86 platforms"
- *   by Agner Fog
- *
- * The latter contains a 32-bit variant of the same algorithm coded in assembly for i386.
- */
-ENTRY(strlen)
-	PUSH_FRAME_POINTER
-	movabsq	$0xfefefefefefefeff,%r8
-	movabsq	$0x8080808080808080,%r9
-
-	movq	%rdi,%r10
-	movq	%rdi,%rcx
-	testb	$7,%dil
-	jz	2f
-
-	/*
-	 * Handle misaligned reads: align to 8 and fill
-	 * the spurious bytes.
-	 */
-	andq	$~7,%rdi
-	movq	(%rdi),%r11
-	shlq	$3,%rcx
-	movq	$-1,%rdx
-	shlq	%cl,%rdx
-	notq	%rdx
-	orq	%rdx,%r11
-
-	leaq	(%r11,%r8),%rcx
-	notq	%r11
-	andq	%r11,%rcx
-	andq	%r9,%rcx
-	jnz	3f
-
-	/*
-	 * Main loop.
-	 */
-	ALIGN_TEXT
-1:
-	leaq	8(%rdi),%rdi
-2:
-	movq	(%rdi),%r11
-	leaq	(%r11,%r8),%rcx
-	notq	%r11
-	andq	%rcx,%r11
-	andq	%r9,%rcx
-	jz	1b
-3:
-	bsfq	%rcx,%rcx
-	shrq	$3,%rcx
-	leaq	(%rcx,%rdi),%rax
-	subq	%r10,%rax
-	POP_FRAME_POINTER
-	ret
-END(strlen)
-
 /*****************************************************************************/
 /* copyout and fubyte family                                                 */
 /*****************************************************************************/

sys/conf/files.amd64

@@ -390,6 +390,7 @@ isa/syscons_isa.c		optional	sc
 isa/vga_isa.c			optional	vga
 kern/imgact_aout.c		optional compat_aout
 kern/link_elf_obj.c		standard
+libkern/strlen.c		standard
 #
 # IA32 binary support
 #