c: Turn -Wimplicit-function-declaration into a permerror: Fix 'gcc.dg/gnu23-builtins...

c: Turn -Wimplicit-function-declaration into a permerror: Fix 'gcc.dg/gnu23-builtins-no-dfp-1.c'

With recent commit 55e94561e97ed0bce4774aa1c6b5d5d82209a379
"c: Turn -Wimplicit-function-declaration into a permerror", this test
case, added in 2019 commit 5b8d9367684f266c30c280b4d3c98830a88c70ab
"Prevent all uses of DFP when unsupported (PR c/91985)" started FAILing
(for applicable configurations):

    [-PASS:-]{+FAIL:+} gcc.dg/gnu23-builtins-no-dfp-1.c  (test for warnings, line 13)
    [-PASS:-]{+FAIL:+} gcc.dg/gnu23-builtins-no-dfp-1.c  (test for warnings, line 14)
    [-PASS:-]{+FAIL:+} gcc.dg/gnu23-builtins-no-dfp-1.c  (test for warnings, line 15)
    [-PASS:-]{+FAIL:+} gcc.dg/gnu23-builtins-no-dfp-1.c  (test for warnings, line 16)
    [-PASS:-]{+FAIL:+} gcc.dg/gnu23-builtins-no-dfp-1.c  (test for warnings, line 17)
    [-PASS:-]{+FAIL:+} gcc.dg/gnu23-builtins-no-dfp-1.c  (test for warnings, line 18)
    [-PASS:-]{+FAIL:+} gcc.dg/gnu23-builtins-no-dfp-1.c (test for excess errors)

This is due to:

    [...]/gcc.dg/gnu23-builtins-no-dfp-1.c:13:13: error: implicit declaration of function '__builtin_fabsd32'; did you mean '__builtin_fabsf32'? [-Wimplicit-function-declaration]
    [...]

Specifying '-fpermissive', commit f37744662cbc74efcceb790b99dcd6521c51a578
"[committed] Fix gnu23-builtins-no-dfp" subsequently resolved the FAILs, but
patch review concluded that for this test case it's secondary *how*
"implicit declaration of function" is diagnosed, so we'd test the standard
way, which instead of "warning" now is "error".

	gcc/testsuite/
	* gcc.dg/gnu23-builtins-no-dfp-1.c: Remove '-fpermissive'.
	'dg-error "implicit"' instead of 'dg-warning "implicit"'.