[Patch] Fix filebuf_virtuals.cc for unsigned char platforms

[Paolo Carlini]

Mark Wielaard wrote:

>On powerpc-unknown-linux-gnu without the patch compiling this test
>gives:
>
>/home/mark/src/gcc33/gcc/libstdc++-v3/testsuite/27_io/filebuf_virtuals.cc: In static member function `static bool MyTraits::eq(char, char)':
>/home/mark/src/gcc33/gcc/libstdc++-v3/testsuite/27_io/filebuf_virtuals.cc:545: warning: comparison is always true due to limited range of data type
>/home/mark/src/gcc33/gcc/libstdc++-v3/testsuite/27_io/filebuf_virtuals.cc:546: warning: comparison is always true due to limited range of data type
>
>With the patch applied it compiles cleanly.
>This was on the 3.3 branch and compiled by hand.
>
My suspects are _fully_ confirmed  then...

>The machine is busy doing a 3.4 bootstrap, I have applied the patch to
>the test and the results should show up on gcc-testresults in a couple
>of hours.
>
Thank you very much!

Paolo.