Use boolean literals in linux-nat.c

Use boolean literals in linux-nat.c

I noticed a couple of spots in linux-nat.c that use 0/1 where boolean
literals would be more idiomatic.  This patch makes this change.

gdb/ChangeLog
2020-07-17  Tom Tromey  <tromey@adacore.com>

	* linux-nat.c (linux_nat_target::supports_non_stop)
	(linux_nat_target::always_non_stop_p): Use "true".
	(linux_nat_target::supports_disable_randomization): Use "true" and
	"false".