Add a if-exists-then-else built-in spec function

Add a if-exists-then-else built-in spec function

This patch adds an if-exists-then-else
builtin spec function, which tests for the existence of
a file and returns one or the other of the following
arguments depending on the result of the test.

This differs from the existing if-exists or
if-exists-else function which return the name of the
tested file if it exists.

This new function is of help to a forthcoming change for
VxWorks where we check for the presence of a specific header
file to decide the name of a library to include in the link
closure.

2020-10-23  Douglas Rupp  <rupp@adacore.com>

gcc/
	* gcc.c (if-exists-then-else): New built-in spec function.
	* doc/invoke.texi: Document it.