[lld-macho] Set 4-byte alignment for `__init_offsets`

[BertalanD]

Summary

dyld refuses to run initializers if this section is not aligned.

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=1383240

[int3]

lgtm

lld/MachO/SyntheticSections.cpp

void ObjCImageInfoSection::writeTo(uint8_t *buf) const {
   uint32_t flags = info.hasCategoryClassProperties ? 0x40 : 0x0;
   flags |= info.swiftVersion << 8;
   write32le(buf + 4, flags);
 }
 
 InitOffsetsSection::InitOffsetsSection()
     : SyntheticSection(segment_names::text, section_names::initOffsets) {
   flags = S_INIT_FUNC_OFFSETS;
+  align = 4; // This section contains 32-bit integers.
 }
 
 uint64_t InitOffsetsSection::getSize() const {
   size_t count = 0;
   for (const ConcatInputSection *isec : sections)
     count += isec->relocs.size();
   return count * sizeof(uint32_t);
 }

lld/test/MachO/init-offsets.s

 ## - for undefined and dylib symbols, stubs are created, and the offsets point to those.
 ## - offsets are relative to __TEXT's address, they aren't an absolute virtual address.
 
 # FLAGS:      sectname __init_offsets
 # FLAGS-NEXT:  segname __TEXT
 # FLAGS-NEXT:     addr
 # FLAGS-NEXT:     size 0x0000000000000010
 # FLAGS-NEXT:   offset
-# FLAGS-NEXT:    align
+# FLAGS-NEXT:    align 2^2 (4)
 # FLAGS-NEXT:   reloff 0
 # FLAGS-NEXT:   nreloc 0
 # FLAGS-NEXT:     type S_INIT_FUNC_OFFSETS
 
 # CONTENT:      segname __TEXT
 # CONTENT-NEXT: 0x[[#%x, TEXT:]]
 
 # CONTENT:      Contents of (__TEXT,__stubs) section