c++: Fix g++.dg/ext/attr-section2.C etc. with Solaris/SPARC as

The new g++.dg/ext/attr-section2*.C tests FAIL on Solaris/SPARC with the
native assembler:

+FAIL: g++.dg/ext/attr-section2.C -std=c++14 scan-assembler
.(section|csect)[ \\\\t]+.foo
+FAIL: g++.dg/ext/attr-section2.C -std=c++17 scan-assembler
.(section|csect)[ \\\\t]+.foo
+FAIL: g++.dg/ext/attr-section2.C -std=c++20 scan-assembler
.(section|csect)[ \\\\t]+.foo

The problem is that the SPARC assembler requires the section name to be
double-quoted, like

        .section        ".foo%_Z3varIiE",#alloc,#write,#progbits

This patch allows for that.  At the same time, it quotes literal dots in
the REs.

Tested on sparc-sun-solaris2.11 (as and gas) and i386-pc-solaris2.11 (as
and gas).

2024-01-18  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/testsuite:
	* g++.dg/ext/attr-section2.C (scan-assembler): Quote dots.  Allow
	for double-quoted section name.
	* g++.dg/ext/attr-section2a.C: Likewise.
	* g++.dg/ext/attr-section2b.C: Likewise.