[mach-o] fix typos. No function change
[mach-o] enhance arm64 reloc parser to support local pointer relocations

The arm64 assembler almost always uses r_extern=1 relocations in which the
r_symbolnum field is the index of the symbol the relocation references.  But
sometimes it will set r_extern=0 in which case the linker needs to read the
content of the reloction to determine the target.

Add test case that the r_extern=0 relocation round trips.
[mach-o] fix typos. No function change
[mach-o] enhance arm64 reloc parser to support local pointer relocations

The arm64 assembler almost always uses r_extern=1 relocations in which the
r_symbolnum field is the index of the symbol the relocation references.  But
sometimes it will set r_extern=0 in which case the linker needs to read the
content of the reloction to determine the target.

Add test case that the r_extern=0 relocation round trips.
[mach-o] extract atom alignment information from mach-o files
[mach-o] zero-fill sections have no file offset

In mach-o, sections of type S_ZEROFILL are special cased and to always have
their section.offset field be zero.