Add dynamic memory allocator support for nolibc

Hi,

This is the v2 of RFC to add dynamic memory allocator support for
nolibc.

## Background
The need to allocate memory dynamically has become a requirement for
the C programming language. Mainly it happens when the allocation size
is determined at runtime. Many other use cases also do it when the
object's lifetime is long-lived and needs to be recycled at runtime.
Currently, the nolibc header doesn't support such a type of allocation.
This series adds it.

## Implementation
Add basic functions to manage dynamic memory allocation:
  - malloc()
  - calloc()
  - realloc()
  - free()

The allocator uses mmap() syscall to allocate the memory and uses
munmap() syscall to free the allocated memory. The metadata to keep
track the length for munmap-ing is simply defined as a struct below:
```
struct nolibc_heap {
        size_t  len;
        char    user_p[] __attribute__((__aligned__));
};
```
malloc(), realloc() and calloc() return a pointer to `user_p`.

## Add my_syscall6() support for x86 32-bit.
mmap() needs 6 arguments to work with. Not all architectures that
nolibc supports have the my_syscall6() wrapper. This series also
adds my_syscall6() wrapper support for i386.

Notes:
On i386, the 6th argument of syscall goes in %ebp. However, both Clang
and GCC cannot use %ebp in the clobber list and in the "r" constraint
without using -fomit-frame-pointer. To make it always available for
any kind of compilation, the below workaround is implemented.

For clang (the Assembly statement can't clobber %ebp):
  1) Push the 6-th argument.
  2) Push %ebp.
  3) Load the 6-th argument from 4(%esp) to %ebp.
  4) Do the syscall (int $0x80).
  5) Pop %ebp (restore the old value of %ebp).
  6) Add %esp by 4 (undo the stack pointer).

For GCC, fortunately it has a #pragma that can force a specific function
to be compiled with -fomit-frame-pointer, so it can use "r"(var) where
var is a variable bound to %ebp.

## Test
The following simple program can be used to test this series:

  https://gist.github.com/ammarfaizi2/db0af6aa0b95a0c7478bc...

@@ Changelog:

   Link RFC v1: https://lore.kernel.org/lkml/20220320093750.159991-1-amma...
   RFC v1 -> RFC v2:
    - Add 2 new patches [PATCH 5/8] and [PATCH 7/8].

    [PATCH 2/8]
    - Remove all `.global _start` for all build (GCC and Clang) instead of
      removing all `.weak _start` for clang build (Comment from Willy).

    [PATCH 3/8]
    - Fix %ebp saving method. Don't use redzone, i386 doesn't have a redzone
      (comment from David and Alviro).

    [PATCH 6/8]
    - Move container_of() and offsetof() macro to types.h with a
      separate patch (comment from Willy).

    [PATCH 8/8]
    - Update strdup and strndup implementation, use strlen and strnlen to get
      the string length first (comment from Willy and Alviro).
    - Fix the subject line prefix, it was "tools/include/string: ", it should be
      "tools/nolibc/string: ".
    - Update the commit message.

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
Ammar Faizi (8):
  tools/nolibc: x86-64: Update System V ABI document link
  tools/nolibc: Remove .global _start from the entry point code
  tools/nolibc: i386: Implement syscall with 6 arguments
  tools/nolibc/sys: Implement `mmap()` and `munmap()`
  tools/nolibc/types: Implement `offsetof()` and `container_of()` macro
  tools/nolibc/stdlib: Implement `malloc()`, `calloc()`, `realloc()` and `free()`
  tools/nolibc/string: Implement `strnlen()`
  tools/include/string: Implement `strdup()` and `strndup()`

 tools/include/nolibc/arch-aarch64.h |  1 -
 tools/include/nolibc/arch-arm.h     |  1 -
 tools/include/nolibc/arch-i386.h    | 67 +++++++++++++++++++++++++++-
 tools/include/nolibc/arch-mips.h    |  1 -
 tools/include/nolibc/arch-riscv.h   |  1 -
 tools/include/nolibc/arch-x86_64.h  |  3 +-
 tools/include/nolibc/stdlib.h       | 68 +++++++++++++++++++++++++++++
 tools/include/nolibc/string.h       | 41 +++++++++++++++++
 tools/include/nolibc/sys.h          | 62 ++++++++++++++++++++++++++
 tools/include/nolibc/types.h        | 11 +++++
 10 files changed, 249 insertions(+), 7 deletions(-)


base-commit: 45abaed6ed05856556c60863f4cac429c92c431f
-- 
Ammar Faizi