Clear screen via crt0.c (llvm-mos#38)

* Minimal OSI C1P implementation

All boilerplate except for the custom _exit implementation
that jumps to the PROM monitor prompt.

* __putchar() for OSI target

* Remove obsolete comment

* Switch to loop exit implementation

This does make more sense with the existing examples that
otherweise would jump directly to the PROM monitor.

* Switch to exit-loop implementation

The endless  loop in the abort implementation is
obsolete with the switch to the common-exit-loop
implementation.

* Address review comments

Use ' ' consistently for space character.
Fix incorrect encoding for CR and LF.

* C standard semantics of '\n'

* Attempt to implement crt0.c

This is a first broken attempt to hook up a call for clearing the screen
before entering main.

* Missing newline

* Fix problem with crt0.c

The CMake call for merge_libraries for common-crt0-o was missing.
This caused an invalid executable being generated where main itself
was missing.

* Adapt to new library merging strategy

It is no longer necessary to merge the crt0.o module explicitly
with the parent library crt0.o module.

* Fix incorrect argument for memset()

Fixed the bug that the address of function scr_base was passed
to memset() instead of calling scr_base() and passing the
result to memset().

* Fix C++ namespace pollution

Fix pollution of C++ namespace by class name that is
not prefixed by underscores.

Fixes llvm-mos#39.

* Simplify clear screen in crt0

Replaced crt0 C module containing inline assembly with
plain assembler module where __clrscr is called directly
without involving an additional C function.

Add workaround for -1 offset problem.

Set rc0; don't provide.

Refactor iNES header.

Install ppu.h; reference using angle brackets.

Simplify linker scripts using custom output formats.

This takes advantage of a new llvm-mos LLD feature to directly define
the output format in terms of header bytes and loaded memory regions.

Simplify library handling.

This uses library merging to join together dependent libraries. This
allows the common libraries to be a real set of targets; this in turn
means it's easy to make an ad-hoc target, then later bless it into a
real one shipped with the SDK.

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