cmd/go: reject cgo -sectcreate linker flags by default

[carrerasdarren-cell]

The Darwin linker -sectcreate option reads a file and embeds its contents.
The existing cgo linker allowlist accepts this option, so a dependency can
make go build read an arbitrary file available to the build process.

Reject file-backed -Wl,-sectcreate,... flags in package-provided #cgo
LDFLAGS by default. Preserve the established
-Wl,-sectcreate,__RESTRICT,__restrict,/dev/null hardening form because it
does not expose useful file contents. Trusted uses can explicitly opt in
with CGO_LDFLAGS_ALLOW, as with other rejected flags.

Add a script test for default rejection, explicit opt-in, and the preserved
hardening form.

Fixes #78750.

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Ian Lance Taylor:

Patch Set 2:

(1 comment)

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[gopherbot]

This PR (HEAD: 26dcf14) has been imported to Gerrit for code review.

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