In this PR I'm changing the way we provide the missing functions like
strnlen() on z/OS from the separate header file to a wrapper around the
system headers that declare these functions. This will be less
intrusive.

---------

Co-authored-by: Zibi Sarbinowski <zibi@ca.ibm.com>

yaml2obj would crash when processing Mach-O load commands with cmdsize
smaller than the actual structure size e.g. LC_SEGMENT_64 with
cmdsize=56 instead of 72. The crash occurred due to integer underflow
when calculating padding: cmdsize - BytesWritten wraps to a large value
when negative, causing a massive allocation attempt.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

Identified with misc-include-cleaner.

This patch adds strnlen to the zOSSupport.h file to fix build failures in multiple files.

Differential Revision: https://reviews.llvm.org/D139487

Use big obj copy in range for-loop will call copy constructor every time,
which can be avoided by use ref instead.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D150024

Before:
```
wrote too much data somewhere, section offsets don't line up
```

After:
```
wrote too much data somewhere, section offsets in section __debug_str for segment __DWARF don't line up: [cursor=0x581], [fileStart=0x0], [sectionOffset=0x579]
```

Differential Revision: https://reviews.llvm.org/D140116