Avoid parallel GCC install in build-many-glibcs.py.

Avoid parallel GCC install in build-many-glibcs.py.

A recent build-many-glibcs.py build
<https://sourceware.org/ml/libc-testresults/2017-q1/msg00067.html> ran
into what proves to be an old known bug
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42980> with parallel
install of GCC (one which as discussed there might require automake
changes to fix).  This patch makes build-many-glibcs.py avoid such
intermittent failures from parallel install by using -j1 for GCC make
install (the code in question also applies to binutils make install,
but it doesn't seem worth trying to avoid -j1 there; the builds and
installs of different toolchains are still fully parallel with each
other, this is only about the case when there are few enough of those
that multiple jobs can get used within a single make install).

	* scripts/build-many-glibcs.py (Config.build_cross_tool): Use -j1
	for make install.