add support for --disable-gcov

add support for --disable-gcov

For some targets (in my case VxWorks 5.5), libgcov does not compile due
to missing functions and macros such as getpid() and F_OK.

Incidentally, gcc/Makefile.in already contains comments such as

# Install gcov if it was compiled.

but there is no logic in place to actually allow gcov to not be
compiled.

So add an option for disabling build and install of libgcov and the
related host tools.

From-SVN: r262180