[testsuite] clean up GCC_EXEC_PREFIX use for PR38526

[John David Anglin]

> > Running ./foo.exp ...
> > good: GCC_EXEC_PREFIX=3Dfoo
> > bad: GCC_EXEC_PREFIX=3D
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> 
> That is the problem. unsetenv doesn't work on Darwin.

unsetenv is implemented in utils.exp.  This appears to rely on
the tcl proc unset.  I'm using the stock darwin version of tcl:

dave@john-david-anglins-mac-pro:~$ runtest --version
WARNING: Couldn't find the global config file.
Expect version is	5.42.0
Tcl version is		8.4
Framework version is	1.4.4

The bash shell doesn't appear to have this problem.

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)