PR gas/12198

	PR gas/12198
	* gas/config/tc-arm.c (arm_arch_v6m_only): New variable.
	(aeabi_set_public_attributes): Ensure we only set the Operating System
	Extension when we are on an M-profile core.
	* gas/testsuite/gas/arm/pr12198-1.d: New test.
	* gas/testsuite/gas/arm/pr12918-1.s: Likewise.
	* gas/testsuite/gas/arm/pr12198-2.d: Likewise.
	* gas/testsuite/gas/arm/pr12918-2.s: Likewise.
	* include/opcode/arm.h (ARM_AEXT_V6M_ONLY): New define.
	(ARM_AEXT_V6M): Rewrite in terms of ARM_AEXT_V6M_ONLY.
	(ARM_ARCH_V6M_ONLY): New define.