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

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.

Reviewed By: ayermolo

Differential Revision: https://reviews.llvm.org/D148217