all: add plan9/arm64 port

[rafael2k]

This PR adds support for the plan9/arm64 platform.

Work adapted previous work from @psilva261 https://github.com/psilva261/go-arm64.plan9

Implements #76272

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Message from Rafael Diniz:

Patch Set 15:

Thanks again for the detailed review. Summary of where PS15 stands, point by point:

1. Compiler-heap corruption (initIntrinsics / sha256 garbage / 'intrinsic already exists on s390x'): I think I found the mechanism. plan9/arm64 was the only Plan 9 port returning heap pages to the OS during sysUnused, via segfree (mem_plan9_arm64.go). Dropping a page's physical backing that way is unsafe under SMP unless the kernel guarantees a cross-core TLB shootdown: another core can keep a stale mapping and read/write the page after it's recycled, corrupting unrelated live allocations. In PS15 sysUnusedOS is now a no-op on plan9/arm64, matching plan9/{386,arm,amd64}; the segment reservation is kept. I want to be upfront about my testing: I can only run under QEMU/TCG, and it does NOT reproduce this - the exact packages from your crash logs, go build -a -p 4, ran 5 clean fresh-cache iterations on 4 emulated cores even WITHOUT the fix. So TCG isn't exercising the TLB path, consistent with your point that emulation won't reproduce multicore behaviour. I therefore cannot claim from my side that the panics are gone - could you retest PS15 on real hardware?

2. Disabling tests that pass on Plan 9 / 9legacy (PS13): you were right, and I've reverted all of them. PS15 no longer touches crypto/x509/root_plan9.go, net/{lookup,timeout,writev}_test.go, or runtime/{goroutineleakprofile,mgcscavenge,mpagealloc,mpagecache}_test.go. Where I'd seen failures they traced to the 9front loopback tcpsplice kernel bug (fixed 9front-side) and the segfree corruption above, not to anything needing a waiver.

3. ABI/timesplit/walltime (PS3, PS9): fixed. open/dupfd now 8-byte-align the int32 syscall args as you described; timesplit uses the simpler arm64 UDIV/MUL/SUB form you suggested (sec+8/nsec+16); walltime is a Go function again. go vet -asmdecl runtime and syscall are both clean.

4. Real SMP hardware (PS14): honestly, so far only QEMU/TCG, which as you noted won't model MMU/TLB/cache/reordering - the segfree bug is a concrete instance of exactly that. I don't yet have physical multicore arm64 results; I'll arrange hardware testing and won't ask for another substantive review until I can report real-hardware results.

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