testsuite: Allow general skips/requires in PCH tests

dg-pch.exp handled dg-require-effective-target pch_supported_debug
as a special case, by grepping the source code.  This patch tries
to generalise it to other dg-require-effective-targets, and to
dg-skip-if.

There also seemed to be some errors in check-flags.  It used:

    lappend $args [list <elt>]

which treats the contents of args as a variable name.  I think
it was supposed to be "lappend args" instead.  From the later
code, the element was supposed to be <elt> itself, rather than
a singleton list containing <elt>.

We can also save some time by doing the common early-exit first.

Doing this removes the need to specify the dg-require-effective-target
in both files.  Tested by faking unsupported debug and checking that
the tests were still correctly skipped.

gcc/testsuite/
	* lib/target-supports-dg.exp (check-flags): Move default argument
	handling further up.  Fix a couple of issues in the lappends.
	Avoid frobbing the compiler flags if the return value is already
	known to be 1.
	* lib/dg-pch.exp (dg-flags-pch): Process the dg-skip-if and
	dg-require-effective-target directives to see whether the
	assembly test should be skipped.
	* gcc.dg/pch/valid-1.c: Remove dg-require-effective-target.
	* gcc.dg/pch/valid-1b.c: Likewise.

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