Modula-2: Support '-isysroot [...]'

Modula-2: Support '-isysroot [...]'

In GCC cross configurations (tested '--target=amdgcn-amdhsa' and
'--target=nvptx-none') with a sysroot configured, the 'gm2' driver invocations
are passed '--sysroot=[...]', which is translated into '-isysroot [...]' for
the 'cc1gm2' compiler invocation.  The latter, however gets complained about:

    cc1gm2: warning: command-line option ‘-isysroot [...]’ is valid for C/C++/D/Fortran/ObjC/ObjC++ but not for Modula-2

..., and therefore a ton of FAILs.

Reproducer (also for non-cross, native configurations):

    $ build-gcc/gcc/gm2 -Bbuild-gcc/gcc -v --sysroot=/tmp -x modula-2 /dev/null
    [...]
     build-gcc/gcc/cc1gm2 [...] -isysroot [...]/tmp [...]
    cc1gm2: warning: command-line option ‘-isysroot /tmp’ is valid for C/C++/D/Fortran/ObjC/ObjC++ but not for Modula-2
    [...]

	gcc/m2/
	* lang.opt (-isysroot): New.