aarch64: Make spmzr_el0 write-only

aarch64: Make spmzr_el0 write-only

Remove all test cases that expect spmzr_el0 to be readable, and remove
some redundant default macro values from armv9_5-a-sysregs.s while we're
there.

Add a read of spmzr_el0 to sysreg-diagnostics.s.  This turns out to be
the first test for the "reading from a write-only register" note.
Also remove the recently added -menable-sysreg-checking option from this
test, both to simplify the addition of spmzr_el0 to the test, and to
verify that read/write diagnostics don't depend on that option.