testsuite: fix allocator-opt1.C FAIL with old ABI

testsuite: fix allocator-opt1.C FAIL with old ABI

Running
$ make check-g++ RUNTESTFLAGS='--target_board=unix\{-D_GLIBCXX_USE_CXX11_ABI=0,\} dg.exp=allocator-opt1.C'
yields:

FAIL: g++.dg/tree-ssa/allocator-opt1.C  -std=c++98  scan-tree-dump-times gimple "struct allocator D" 1
FAIL: g++.dg/tree-ssa/allocator-opt1.C  -std=c++14  scan-tree-dump-times gimple "struct allocator D" 1
FAIL: g++.dg/tree-ssa/allocator-opt1.C  -std=c++17  scan-tree-dump-times gimple "struct allocator D" 1
FAIL: g++.dg/tree-ssa/allocator-opt1.C  -std=c++20  scan-tree-dump-times gimple "struct allocator D" 1

		=== g++ Summary for unix/-D_GLIBCXX_USE_CXX11_ABI=0 ===

		=== g++ Summary for unix ===

because in the old ABI we get two "struct allocator D".  This patch
follows r14-658 although I'm not quite sure I follow the logic there.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/allocator-opt1.C: Force _GLIBCXX_USE_CXX11_ABI to 1.