ww: import toolchain — C bootstrap + ww-side self-host (phases 0-10)

C bootstrap (phases 0-9):
  cmd/wwc, cmd/6c, cmd/6a, cmd/6l, cmd/ww, rt, lib/*.

ww-side self-host (phase 10):
  selfhost/cmd/wwc — ww-cgen frontend; bootstrap fixed point.
  selfhost/cmd/6a  — assembler; byte-identical to C 6a (test 991).
  selfhost/cmd/6l  — linker w/ archive (.a) support; byte-identical
                     to C 6l (test 992).
  selfhost/cmd/ww  — driver (build/run/version); byte-identical to
                     C ww (test 993).

make test: 15/15. make bootstrap: ww2.s == ww3.s, ww2.o == ww3.o,
ww2 == ww3 byte-identical, with the full ww-tooled chain.

cmd/6l/l.h

@@ -0,0 +1,74 @@
+/*
+ * l.h — 6l-private header. Loads relocatable ELF64 .o files (the
+ * format produced by 6a) and links them into a static executable.
+ *
+ * No archives yet (phase 8). No dynamic linking ever.
+ */
+#ifndef SIX_L_H
+#define SIX_L_H
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+
+typedef int8_t   i8;
+typedef int16_t  i16;
+typedef int32_t  i32;
+typedef int64_t  i64;
+typedef uint8_t  u8;
+typedef uint16_t u16;
+typedef uint32_t u32;
+typedef uint64_t u64;
+
+typedef struct Lsym  Lsym;
+typedef struct Lrel  Lrel;
+typedef struct Lobj  Lobj;
+typedef struct Lnk   Lnk;
+
+struct Lsym {
+	const char *name;
+	u64	val;		/* offset within combined .text once linked */
+	int	defined;	/* 1 if a Lobj defines this symbol */
+	Lobj	*owner;
+	int	idx_in_owner;
+	Lsym	*next;
+};
+
+struct Lrel {
+	u64	off;		/* offset within combined .text */
+	int	kind;		/* R_X86_64_* */
+	Lsym	*sym;
+	i64	addend;
+	Lrel	*next;
+};
+
+struct Lobj {
+	const char *path;
+	u8	*buf;		/* mmapped or read-in object bytes */
+	u64	len;
+	u64	text_off;	/* offset of .text in combined output */
+	u64	text_size;
+	Lobj	*next;
+};
+
+struct Lnk {
+	Lobj	*objs;
+	Lsym	*syms;
+	Lrel	*rels;
+	u8	*text;		/* combined .text */
+	u64	textcap, textlen;
+	int	errs;
+};
+
+/* obj.c */
+int	l_load(Lnk*, const char *path);
+/* sym.c */
+Lsym	*l_intern(Lnk*, const char *name);
+Lsym	*l_lookup(Lnk*, const char *name);
+/* pass.c */
+int	l_resolve(Lnk*);
+int	l_relocate(Lnk*, u64 base);
+/* out.c */
+int	l_emit_elf(Lnk*, FILE *out, u64 base, u64 entry);
+
+#endif