[PATCH doc] use "cannot" consistently

[Martin Sebor]

In formal writing it's recommended to prefer the word "cannot"
to the somewhat informal "can't."

The GCC manual uses "cannot" in most places (280 lines) but there
are a few instances of "can't" (33 lines).

The attached patch replaces the informal "can't" with the former
for consistency.

Thanks
Martin

PS I wasted quite a bit of time updating tm.texi.  I kept getting
the error below and didn't realize (forgot) that it was asking me
to copy $objdir/gcc/tm.texi to $srcdir/gcc/doc/tm.texi.  Can
someone explain why this file requires these special steps when
the rest of the .texi files can be updated directly?

mv tmp2-tm.texi tmp-tm.texi
/bin/sh /src/gcc/svn/gcc/../move-if-change tmp-tm.texi tm.texi

Verify that you have permission to grant a GFDL license for all
new text in tm.texi, then copy it to /src/gcc/svn/gcc/doc/tm.texi.,
Makefile:2437: recipe for target 's-tm-texi' failed
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