[testsuite] clean up GCC_EXEC_PREFIX use for PR38526

[John David Anglin]

> Will commit the following as per your approval on IRC, after
> testing cross to crisv32-axis-linux-gnu and native
> x86_64-unknown-linux-gnu:

Unfortunately, this change re-introduces the problem compiling the
compat generator on darwin:
http://gcc.gnu.org/ml/gcc-testresults/2009-03/msg02935.html

Executing on build: gcc -g -O2 -o /Users/dave/gnu/gcc/objdir/gcc/testsuite/g++/g
++.dg-struct-layout-1_generate.exe /Users/dave/gnu/gcc/gcc/gcc/testsuite/g++.dg/
compat/struct-layout-1_generate.c /Users/dave/gnu/gcc/gcc/gcc/testsuite/g++.dg/c
ompat/../../gcc.dg/compat/generate-random.c /Users/dave/gnu/gcc/gcc/gcc/testsuit
e/g++.dg/compat/../../gcc.dg/compat/generate-random_r.c    (timeout = 300)
gcc: error trying to exec 'cc1': execvp: No such file or directory
gcc: error trying to exec 'cc1': execvp: No such file or directory
gcc: error trying to exec 'cc1': execvp: No such file or directory

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