aarch64: Fix w iterator for CADImode

aarch64: Fix w iterator for CADImode

When the w iterator was initially extended to CADImode, there was a
slight bodge in using the empty string for CADImode. This conveniently
works for REG rtxes (and even does the right thing for fakecap) but
fails for e.g. const_null (as the testcase shows: the atomic_store<mode>
pattern runs into this, as it stands).

To fix this properly, this patch adds the 'B' format specifier to
aarch64_print_operand (for capaBility: unfortunately 'c' is already
taken) which forces the operand to be printed as a c-register, except
for fakecap where it emits an x-register.

This fixes the bug hit by the new test and hopefully any other pattern
using the w iterator. Other patterns may later need updating to use the
B prefix where they expect to take const_null operands.

gcc/ChangeLog:

	* config/aarch64/aarch64.c (aarch64_print_operand): Implement
	'B' for capaBility.
	* config/aarch64/iterators.md (w): Map CADImode -> "B".

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/morello/atomic-store-asm.c: New test.