internal/runtime/gc/scan: fix calls for retpoline compatibility

[GeorgeSapkin]

The assembler's retpoline pass for Spectre mitigations did not recognize
several indirect CALL instructions, causing assembly failures with:

# internal/runtime/gc/scan
asm: non-retpoline-compatible: 00005 (/path_to_go/src/internal/runtime/gc/scan/scan_amd64.s:15) CALL (BX)(CX8)
asm: non-retpoline-compatible: 00015 (/path_to_go/src/internal/runtime/gc/scan/scan_amd64.s:28) CALL (BX)(CX8)
asm: assembly failed
# internal/runtime/startlinetest
asm: non-retpoline-compatible: 00005 (/path_to_go/src/internal/runtime/startlinetest/func_amd64.s:27) CALL (DX)
asm: assembly failed

Adapt the calls in func_amd64.s and scan_amd64.s to allow the assembler to
correctly apply retpolines and fix the build with Spectre mitigations
enabled.

I'm not certain about the syntax or the exact registers (e.g. R11), but
go tool dist test passes on target. There's probably a better approach.

Commits are split in case this needs backporting, since paths are different
on <1.26.

Fixes #77420

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[mp4245]

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