make: do not expand the variable name in the ':_' modifier

This edge case had been so obscure that even discovering this takes
quite some time and requires reading the source code of make.

The manual page doesn't document whether the variable name is expanded
or not, it doesn't even give an example.  When this obscure modifier was
initially added in var.c 1.210 from 2017-01-30, Var_Set always expanded
the variable name once, and there was no way around it.  Therefore this
expansion has probably been unintentional.

(rillig)