[BOLT][NFC] Fix UB due to unaligned load in DebugStrOffsetsWriter

[jobnoorman]

Summary

The following tests fail when enabling UBSan due to an unaligned memory
load:

runtime error: load of misaligned address 0x620000000643 for type
'const uint32_t' (aka 'const unsigned int'), which requires 4 byte
alignment

BOLT :: AArch64/asm-func-debug.test
BOLT :: AArch64/update-debug-reloc.test
BOLT :: X86/asm-func-debug.test
BOLT :: X86/dwarf5-df-dualcu.test
BOLT :: X86/dwarf5-df-mono-dualcu.test
BOLT :: X86/dwarf5-ftypes-dwp-input-dwo-output.test
BOLT :: X86/dwarf5-locaddrx.test
BOLT :: X86/dwarf5-split-dwarf4-monolithic.test
BOLT :: X86/inlined-function-mixed.test
BOLT :: non-empty-debug-line.test

This patch fixes this by using read32le for the load.

[ayermolo]

Thanks for catching it.

bolt/lib/Core/DebugData.cpp

void DebugStrOffsetsWriter::initialize(
   if (!Contr)
     return;
 
   const uint8_t DwarfOffsetByteSize = Contr->getDwarfOffsetByteSize();
   assert(DwarfOffsetByteSize == 4 &&
          "Dwarf String Offsets Byte Size is not supported.");
   uint32_t Index = 0;
   for (uint64_t Offset = 0; Offset < Contr->Size; Offset += DwarfOffsetByteSize)
-    IndexToAddressMap[Index++] = *reinterpret_cast<const uint32_t *>(
+    IndexToAddressMap[Index++] = support::endian::read32le(
         StrOffsetsSection.Data.data() + Contr->Base + Offset);
 }
 
 void DebugStrOffsetsWriter::updateAddressMap(uint32_t Index, uint32_t Address) {
   assert(IndexToAddressMap.count(Index) > 0 && "Index is not found.");
   IndexToAddressMap[Index] = Address;
   StrOffsetSectionWasModified = true;
 }