testsuite: xfail c-c++-common/pr103798-2.c for C++ on Solaris [PR113706]

c-c++-common/pr103798-2.c FAILs on Solaris when compiled as C++:

FAIL: c-c++-common/pr103798-2.c  -std=gnu++14  scan-assembler-not memchr
FAIL: c-c++-common/pr103798-2.c  -std=gnu++17  scan-assembler-not memchr
FAIL: c-c++-common/pr103798-2.c  -std=gnu++20  scan-assembler-not memchr
FAIL: c-c++-common/pr103798-2.c  -std=gnu++98  scan-assembler-not memchr

As Jason analyzed, Solaris <string.h> declares memchr for C++ as returning
const void * as specified by the C++ standard, while gcc expects the return
type to be void * like in C.

So this patch xfails the test for C++ on Solaris.

Tested on sparc-sun-solaris2.11 and x86_64-pc-linux-gnu.

2024-02-12  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/testsuite:
	PR c++/113706
	* c-c++-common/pr103798-2.c (scan-assembler-not): xfail for C++ on
	Solaris.