Cast char constant to int in sizeof.exp

Cast char constant to int in sizeof.exp

PR gdb/22010 concerns a regression I introduced with the scalar
printing changes.  The bug is that this code in sizeof.exp:

    set signof_byte [get_integer_valueof "'\\377'" -1]

can incorrectly compute sizeof_byte.  One underlying problem here is
that gdb's C parser doesn't treat a char constant as an int (this is
PR 19973).

However, it seems good to have an immediate fix for the regression.
The simplest is to cast to an int here.

testsuite/ChangeLog
2017-09-05  Tom Tromey  <tom@tromey.com>

	PR gdb/22010:
	* gdb.base/sizeof.exp (check_valueof): Cast char constant to int.