ld64.lld: Support synthesizing relative ObjC method lists

[BertalanD]

Starting with macOS 11, a more compact encoding is available for ObjC method lists. Previously, each method had a descriptor consisting of 3 pointers:

struct _objc_method {
    SEL _cmd;
    char *method_type;
    char *_imp;
}

In the new format, __TEXT,__objc_methlist stores 32-bit offsets instead.

struct _objc_method_rel {
    int32_t _cmd;
    int32_t method_type;
    int32_t _imp;
}

This transformation can be controlled with the -objc_relative_method_lists/-no_objc_relative_method_lists linker flags.

Currently, method lists are stored in __DATA,__objc_const. With this transformation, Chromium Framework's data segment will shrink by about 40 KB, meaning that 3 fewer pages will have to be relocated at startup.

ref #48630

Sidenote: AppleClang itself can be coaxed into generating the relative encoding with `-fobjc-relative-method-lists`

// test.m
#include <Foundation/Foundation.h>

int main() {
    NSObject *foo = [[NSObject alloc] init];
}

❯ clang test.m  -S -emit-llvm  -o - | grep _objc_method
%struct.__method_list_t = type { i32, i32, [0 x %struct._objc_method] }
%struct._objc_method = type { i8*, i8*, i8* }

❯ clang test.m  -S -emit-llvm  -o - -fobjc-relative-method-lists | grep _objc_method
%struct.__method_list_t = type { i32, i32, [0 x %struct._objc_method_rel] }
%struct._objc_method_rel = type { i32, i32, i32 }

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