mk/compiler/gcc.mk: search for GCCBASE/bin/gcc as well
A default installation of GCC doesn't install bin/cc but only bin/gcc.
Adding bin/cc is only done by the pgksrc packages, not by the upstream
package.
The previous strategy of just checking whether ${GCCBASE}/bin/${CC:[1]}
exists did not work in such a situation. Therefore, if CC still has its
default value from sys.mk, that is changed to the intended gcc, which
then detects the base GCC properly.
See https://mail-index.netbsd.org/pkgsrc-users/2019/09/07/msg029329.html.
See https://mail-index.netbsd.org/tech-pkg/2019/09/18/msg021976.html.
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