`z/OS` archives use the same structural layout as traditional Unix
archives but encode all text fields in EBCDIC. The magic string is the
EBCDIC representation of `\"!<arch>\n\" (hex: 5A 4C 81 99 83 88 6E 15)`.
This patch adds recognition of the `z/OS` archive magic to
`identify_magic()` and defines the `ZOSArchiveMagic` constant. This is
the first in a series of patches adding `z/OS` archive support to LLVM.
3d0e7e04c8