missing -Wformat-overflow with %s and non-member array arguments 

-Wformat-overflow uses the routine "get_range_strlen" to decide the 
maximum string length, however, currently "get_range_strlen" misses 
the handling of non-member arrays.

Adding the handling of non-member array resolves the issue.
Adding test case pr79538.c into gcc.dg.

During gcc bootstrap, 2 source files (c-family/c-cppbuiltin.c, 
fortran/class.c) were detected new warnings by -Wformat-overflow
due to the new handling of non-member array in "get_range_strlen".
in order to avoid these new warnings and continue with bootstrap,
updating these 2 files to avoid the warnings.

in c-family/c-cppbuiltin.c, the warning is following:

../../latest_gcc_2/gcc/c-family/c-cppbuiltin.c:1627:15: note: 
‘sprintf’ output 2 or more bytes (assuming 257) into a destination 
of size 256
       sprintf (buf1, "%s=%s", macro, buf2);
       ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
in the above, buf1 and buf2 are declared as:
char buf1[256], buf2[256];

i.e, buf1 and buf2 have same size. adjusting the size of buf1 and 
buf2 resolves the warning. 

fortran/class.c has the similar issue as above. Instead of adjusting
size of the buffers, replacing sprintf with xasprintf is a better 
solution for these cases.

From-SVN: r255654