Make guality.h handle Yama restricted ptrace mode

guality.exp is silently skipped on Linux systems with
kernel.yama.ptrace_scope=1 because gdb fails to attach to the
sanity check test.  This patch uses PR_SET_PTRACER (where available)
to avoid this.

prctl was apparently added in Linux 2.1.57, so I don't think we
need any tests other than __linux for the #include.

2019-07-08  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/
	* gcc.dg/guality/guality.h: Include <sys/prctl.h> on Linux targets.
	(main): Use PR_SET_PTRACER where available.

From-SVN: r273192