Windows arm64 support

[umlaeute]

I've been playing a bit with building Pd and externals for the Windows/arm64 platform (Using MSYS2's preliminary arm64 support).

AFAICT things run smoothly (barring that the aarch64 VM is emulated somewhat slowly on an x86_64 host), except for pd-lib-builder misdetecting the target system as Linux.

I figure this is due to the unexpected value of the target.triplet:

$ make PDDIR="${PDDIR}" vars | grep -E "(target.triplet|system) ="
variable system = Linux
variable target.triplet = aarch64 w64 windows gnu

$ cc -dumpmachine
aarch64-w64-windows-gnu

$ cc --version
clang version 19.1.6
Target: aarch64-w64-windows-gnu
Thread model: posix
InstalledDir: C:/msys64/clangarm64/bin

$

the problem is obviously that cc -dumpmachine

• does not mention mingw as per

  pd-lib-builder/Makefile.pdlibbuilder

  Lines 433 to 434 in 7752526

  	ifneq ($(filter mingw% cygwin%, $(target.triplet)),)
  	system = Windows

• but does mention gnu as per

  pd-lib-builder/Makefile.pdlibbuilder

  Lines 425 to 426 in 7752526

  	ifneq ($(filter linux gnu% kfreebsd, $(target.triplet)),)
  	system = Linux

The compiler is running natively on Windows (MSYS2/MinGW64), and I have no idea why it doesn't mention mingw.

Also, the dumpmachine information obviously uses 4 sections, so it is strictly speaking not a triplet. but i think this is specific to clang; e.g. on my Linux host clang -dumpmachine returns x86_64-pc-linux-gnu.