Use Windows system headers for POSIX read()/write()/getpid() (rebased #45)

[karypis]

What & why

On Windows, the POSIX read()/write() functions are declared in <io.h> and
getpid() in <process.h>. They are not pulled in by GKlib's Windows include
path, so modern Clang/clang-cl — which treat implicit declarations as hard
errors (-Wimplicit-function-declaration) — fail to compile src/io.c:

src/io.c(63,18): error: call to undeclared function 'read'
src/io.c(84,17): error: call to undeclared function 'write'

This is now reproduced by our own CI (the clang-cl Ninja and -TClangCL
MSBuild jobs) after the CI workflow was added in #46.

Change

• In the __MSC__ branch of include/gk_arch.h, include <io.h> (declares
  read/write) and <process.h> (declares getpid), and typedef int pid_t;.
• Remove the bundled win32/adapt.{c,h} shim, which existed only to provide a
  getpid() wrapper via <windows.h>; <process.h> supplies it directly. The
  one in-tree getpid() caller (src/memory.c) is unaffected.
• Drop the shim from target_sources in CMakeLists.txt.

Net +4 / −30 (a shim removal).

Provenance

This is a rebased version of #45 by @MarijnS95, re-applied onto current master
so it composes with the _MSC_VER >= 1600 stdint guard added by #59 and runs
through the new CI. Original authorship preserved via Co-authored-by.

Validation

• Non-Windows build (cmake -DNO_X86=ON): clean, 0 warnings / 0 errors — the
  #else path is untouched.
• Windows: validated by this PR's CI run (the previously-failing io.c
read/write errors should clear). Other compilers' pre-existing deprecation
  / unused-variable warnings remain but are non-fatal.

Closes #45.