add glue code for mallocng merge

this includes both an implementation of reclaimed-gap donation from
ldso and a version of mallocng's glue.h with namespace-safe linkage to
underlying syscalls, integration with AT_RANDOM initialization, and
internal locking that's optimized out when the process is
single-threaded.

src/malloc/mallocng/README.mallocng

@@ -0,0 +1,13 @@
+This directory is a skeleton for upcoming merge of musl's new malloc
+implementation, mallocng. To use it, drop in copies of or symlinks to
+the following files from mallocng:
+
+- meta.h
+- malloc.c
+- realloc.c
+- free.c
+- aligned_alloc.c
+- malloc_usable_size.c
+
+and build with make variable MALLOC_DIR=mallocng in config.mak or on
+make command line.

src/malloc/mallocng/donate.c

@@ -0,0 +1,39 @@
+#include <stdlib.h>
+#include <stdint.h>
+#include <limits.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <errno.h>
+
+#include "meta.h"
+
+static void donate(unsigned char *base, size_t len)
+{
+	uintptr_t a = (uintptr_t)base;
+	uintptr_t b = a + len;
+	a += -a & (UNIT-1);
+	b -= b & (UNIT-1);
+	memset(base, 0, len);
+	for (int sc=47; sc>0 && b>a; sc-=4) {
+		if (b-a < (size_classes[sc]+1)*UNIT) continue;
+		struct meta *m = alloc_meta();
+		m->avail_mask = 0;
+		m->freed_mask = 1;
+		m->mem = (void *)a;
+		m->mem->meta = m;
+		m->last_idx = 0;
+		m->freeable = 0;
+		m->sizeclass = sc;
+		m->maplen = 0;
+		*((unsigned char *)m->mem+UNIT-4) = 0;
+		*((unsigned char *)m->mem+UNIT-3) = 255;
+		m->mem->storage[size_classes[sc]*UNIT-4] = 0;
+		queue(&ctx.active[sc], m);
+		a += (size_classes[sc]+1)*UNIT;
+	}
+}
+
+void __malloc_donate(char *start, char *end)
+{
+	donate((void *)start, end-start);
+}

src/malloc/mallocng/glue.h

@@ -0,0 +1,77 @@
+#ifndef MALLOC_GLUE_H
+#define MALLOC_GLUE_H
+
+#include <stdint.h>
+#include <sys/mman.h>
+#include <pthread.h>
+#include <unistd.h>
+#include <elf.h>
+#include <string.h>
+#include "atomic.h"
+#include "syscall.h"
+#include "libc.h"
+#include "lock.h"
+#include "dynlink.h"
+
+// use macros to appropriately namespace these.
+#define size_classes __malloc_size_classes
+#define ctx __malloc_context
+#define alloc_meta __malloc_alloc_meta
+#define is_allzero __malloc_allzerop
+#define dump_heap __dump_heap
+
+#if USE_REAL_ASSERT
+#include <assert.h>
+#else
+#undef assert
+#define assert(x) do { if (!(x)) a_crash(); } while(0)
+#endif
+
+#define brk(p) ((uintptr_t)__syscall(SYS_brk, p))
+
+#define mmap __mmap
+#define madvise __madvise
+#define mremap __mremap
+
+#define DISABLE_ALIGNED_ALLOC (__malloc_replaced && !__aligned_alloc_replaced)
+
+static inline uint64_t get_random_secret()
+{
+	uint64_t secret = (uintptr_t)&secret * 1103515245;
+	for (size_t i=0; libc.auxv[i]; i+=2)
+		if (libc.auxv[i]==AT_RANDOM)
+			memcpy(&secret, (char *)libc.auxv[i+1]+8, sizeof secret);
+	return secret;
+}
+
+#ifndef PAGESIZE
+#define PAGESIZE PAGE_SIZE
+#endif
+
+#define MT (libc.need_locks)
+
+#define RDLOCK_IS_EXCLUSIVE 1
+
+__attribute__((__visibility__("hidden")))
+extern int __malloc_lock[1];
+
+#define LOCK_OBJ_DEF \
+int __malloc_lock[1];
+
+static inline void rdlock()
+{
+	if (MT) LOCK(__malloc_lock);
+}
+static inline void wrlock()
+{
+	if (MT) LOCK(__malloc_lock);
+}
+static inline void unlock()
+{
+	UNLOCK(__malloc_lock);
+}
+static inline void upgradelock()
+{
+}
+
+#endif