driver: Try to read spec from gcc_exec_prefix if possible

driver: Try to read spec from gcc_exec_prefix if possible

GCC will try to read the spec file from the directory where it is
installed, but it should try to read from gcc_exec_prefix rather than
standard_exec_prefix, because the latter is not the right one if
compiler has been relocated into other places other than the path
specfied at configuration time.

gcc/ChangeLog:

	* gcc.cc (driver::set_up_specs): Use gcc_exec_prefix to
	read the spec file rather than standard_exec_prefix.