[MachO] Fix unaligned load in extractSections (#68741)

A recent change to ubsan

(https://github.com/llvm/llvm-project/commit/792674400f6f04a074a3827349ed0e2ac10067f6)
exposed an unaligned load in MachOReader (see
https://lab.llvm.org/buildbot/#/builders/85/builds/19482 for an
example).

This patch fixes it by dropping the alignment.