Add hwsync extended mnemonic.

Add hwsync extended mnemonic.

This commit adds a new extended menmonic for "sync 0" (same as "sync").
The ISA documentation doesn't explicitly mention hwsync as an extended
mnemonic (yet), but it does mention "heavyweight sync" and "hwsync" as
the operation that gets performed when the sync's L field is 0.
This is only enabled for POWER4 and later.

opcodes/
	* ppc-opc.c: (powerpc_opcodes) <hwsync>: New extended mnemonic.

gas/testsuite/
	* gas/ppc/a2.d: Fixup test case due to new extended mnemonic.
	* gas/ppc/power4.s <hwsync, lwsync, ptesync, sync>: Add tests.
	* gas/ppc/power4.d: Likewise.