gdb: add missing raw register read in aarch64_sme_pseudo_register_write

gdb: add missing raw register read in aarch64_sme_pseudo_register_write

It seems like the intention here is to read the contents of the ZA
register and only write part of it.  However, there's no actual read of
the ZA register, so it looks like we'll write uninitialized bytes to the
target, for the portion of the raw register where we don't write the
pseudo register.  Add a call to raw_read to fix this.

I don't know how to test this though.

Change-Id: I7548240bd4324f6a3b729a1ebf7502fae5a46e9e
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-by: Luis Machado <luis.machado@arm.com>