[libc] Implement sbrk to allow malloc() to work

Userspace can now allocte via malloc. This is slightly janky because it
relies on a single static handle in the library code.

src/libraries/libc/include/j6libc/syscalls.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <j6/types.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif

src/libraries/libc/j6libc/sbrk.c

@@ -1,4 +1,39 @@
 #include <stdint.h>
-void *sbrk(intptr_t) __attribute__ ((weak));
+#include <j6/errors.h>
+#include <j6libc/syscalls.h>
+//void *sbrk(intptr_t) __attribute__ ((weak));
 
-void *sbrk(intptr_t i) { return 0; }
+static j6_handle_t __core_handle = 0;
+static intptr_t __core_size = 0;
+
+static const uintptr_t __core_base = 0x1c0000000;
+
+static const void *error_val = (void*)-1;
+
+void *sbrk(intptr_t i)
+{
+	if (i == 0)
+		return (void*)__core_base;
+
+	if (__core_size == 0) {
+		if (i < 0)
+			return (void*)-1;
+
+		j6_status_t result = _syscall_vma_create_map(&__core_handle, i, __core_base, 1);
+		if (result != j6_status_ok)
+			return (void*)-1;
+
+		__core_size = i;
+		return (void*)__core_base;
+	}
+
+	size_t new_size = __core_size + i;
+	j6_status_t result = _syscall_vma_resize(__core_handle, &new_size);
+	if (result != j6_status_ok)
+		return (void*)-1;
+
+	uintptr_t prev = __core_base + __core_size;
+	__core_size += i;
+
+	return (void*)prev;
+}