testsuite: Only run test if alarm is available

Most baremetal toolchains will not have an implementation for alarm and
sigaction as they are target specific.
For arm-none-eabi with newlib, function signatures are exposed, but
there is no implmentation and thus the test cases causes a undefined
symbol link error.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr78185.c: Remove dg-do and replace with
	with dg-require-effective-target of signal and alarm.
	* gcc.dg/pr116906-1.c: Likewise.
	* gcc.dg/pr116906-2.c: Likewise.
	* gcc.dg/vect/pr101145inf.c: Use effective-target alarm.
	* gcc.dg/vect/pr101145inf_1.c: Likewise.
	* lib/target-supports.exp(check_effective_target_alarm): New.

gcc/ChangeLog:

	* doc/sourcebuild.texi (Effective-Target Keywords): Document
	'alarm'.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>