Avoid -Wnarrowing warnings in gdbserver

Avoid -Wnarrowing warnings in gdbserver

This avoids -Wnarrowing warnings in gdbserver, by introducing some
casts to unsigned char.

gdb/gdbserver/ChangeLog
2018-08-27  Tom Tromey  <tom@tromey.com>

	* linux-s390-low.c (s390_emit_ext, s390_emit_litpool)
	(s390_emit_const, s390_emit_reg, s390_emit_zero_ext)
	(s390_emit_stack_adjust, s390_emit_set_r2, s390x_emit_ext)
	(s390x_emit_const, s390x_emit_reg, s390x_emit_zero_ext)
	(s390x_emit_stack_adjust): Add casts to unsigned char.