cmd/compile: register conflict between external linker and duffzero on arm64 [1.16 backport]

[gopherbot]

@randall77 requested issue #32773 to be considered for backport to the next 1.16 minor release.

@gopherbot, please open backport issues.

[ianlancetaylor]

Relevant CL: https://golang.org/cl/184437

CC @cherrymui

[ianlancetaylor]

See #46924.

[gopherbot]

Change https://golang.org/cl/331029 mentions this issue: [release-branch.go1.16] cmd/compile: mark R16, R17 clobbered for non-standard calls on ARM64

[toothrot]

Approved. This is a serious issue with no workaround.

[gopherbot]

Closed by merging 8b6ae9b to release-branch.go1.16.