[lld] [lld][MachO] Support for -interposable (PR #131813)

[John Holdsworth via llvm-commits]

johnno1962 wrote:

Modifying the test to use Apple's linker so:
```
# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/2.s -o %t/2.o
# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/3.s -o %t/3.o
# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/main.s -o %t/main.o
# RUN:~/D2/Xcode162.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch x86_64 -isysroot ~/D2/Xcode162.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -Xlinker -interposable -lSystem -o %t/main2 %t/main.o %t/2.o %t/3.o

#--- 2.s
# my_lib: This contains the exported function
.globl my_func
my_func:
  retq

#--- 3.s
# my_user.s: This is the user/caller of the
#            exported function
.text
my_user:
  callq my_func()
  retq

#--- main.s
# main.s: dummy exec/main loads the exported function.
# This is basically a way to say `my_user` should get
# `my_func` from this executable.
.globl _main
.text
 _main:
  retq
```
Yields:
```
Mac-minii:llvm-project$ objdump -d ./build/tools/lld/test/MachO/Output/interposable.s.tmp/main2 

./build/tools/lld/test/MachO/Output/interposable.s.tmp/main2:	file format mach-o 64-bit x86-64

Disassembly of section __TEXT,__text:

0000000100000f9a <_main>:
100000f9a: c3                          	retq

0000000100000f9b <my_func>:
100000f9b: c3                          	retq

0000000100000f9c <my_user>:
100000f9c: e8 01 00 00 00              	callq	0x100000fa2
100000fa1: c3                          	retq

Disassembly of section __TEXT,__stubs:

0000000100000fa2 <__stubs>:
100000fa2: ff 25 58 00 00 00           	jmpq	*0x58(%rip)             ## 0x100001000
```
So it is setting up a stub for an executable. Does that answer your first question?
```
Mac-minii:llvm-project$ file !$
file ./build/tools/lld/test/MachO/Output/interposable.s.tmp/main2
./build/tools/lld/test/MachO/Output/interposable.s.tmp/main2: Mach-O 64-bit executable x86_64


https://github.com/llvm/llvm-project/pull/131813