[PATCH 5/7] testsuite: disable small data for attr-retain tests on alpha

The attr-retain tests scan for the R section flag on .text, .data, .bss
and .rodata.  Alpha places small objects in the gp-relative small data
area instead, emitting .sdata/.sbss with an additional "s" flag, so scans
such as .rodata.*,"aR" do not match even though the retain flag itself is
emitted correctly.

The tests already compile with -G0 on powerpc ilp32 for this reason.  Do
the same on alpha, which fixes 57 failures across the C and C++ testsuites.

gcc/testsuite/ChangeLog:

	* c-c++-common/attr-retain-2.c: Also pass -G0 on alpha.
	* c-c++-common/attr-retain-3.c: Likewise.
	* c-c++-common/attr-retain-4.c: Likewise.
	* gcc.c-torture/compile/attr-retain-1.c: Likewise.
	* gcc.c-torture/compile/attr-retain-2.c: Likewise.