Remove some legacy cruft

Function trace logs will report stack usage accurately. It won't include
the argv/environ block. Our clone() polyfill is now simpler and does not
use as much stack memory. Function call tracing on x86 is now faster too

libc/sysv/BUILD.mk

@@ -84,7 +84,8 @@ o/$(MODE)/libc/sysv/sysret.o: private			\
 		CFLAGS +=				\
 			-ffreestanding			\
 			-fno-stack-protector		\
-			-fno-sanitize=all
+			-fno-sanitize=all		\
+			-mgeneral-regs-only
 
 ifeq ($(ARCH),aarch64)
 o/$(MODE)/libc/sysv/sysv.o: private			\

libc/sysv/errno.c

@@ -35,8 +35,10 @@ errno_t __errno;
 
 /**
  * Returns address of `errno` variable.
+ *
+ * This function promises to not clobber argument registers.
  */
-errno_t *__errno_location(void) {
+nocallersavedregisters errno_t *__errno_location(void) {
   if (__tls_enabled) {
     return &__get_tls()->tib_errno;
   } else {

libc/sysv/systemfive.S

@@ -187,7 +187,7 @@ systemfive_error:
 #endif
 systemfive_errno:
 	xchg	%eax,%ecx
-	.errno
+	call	__errno_location
 	mov	%ecx,(%rax)		// normalize to c library convention
 	push	$-1			// negative one is only error result
 	pop	%rax			// the push pop is to save code size