cmd/go: don't run cgo compiler version probe under -toolexec

[harjothkhara]

The -toolexec build flag wraps each toolchain program (compile, asm,
link, ...) in a user-supplied program, e.g. to profile them or swap in
alternates.

To know when to rebuild a cgo package, the go command records the C,
C++, and Fortran compiler versions by probing each compiler:

cc -### -x c -c -

That probe was run under -toolexec, but the real cgo compiles never
are, so the wrapper saw the probe and none of the actual compiles.

Probe under -toolexec only for gccgo, a Go toolchain compiler treated
like cmd/compile. Leave the C, C++, and Fortran probes unwrapped.

Fixes #64580

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Message from Harjoth Khara:

Patch Set 2:

(1 comment)

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Message from Michael Matloob:

Patch Set 2:

(1 comment)

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Message from Harjoth Khara:

Patch Set 3:

(1 comment)

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Message from Dmitri Shuralyov:

Patch Set 3: Code-Review+1

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Message from Harjoth Khara:

Patch Set 3:

(1 comment)

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