arm64: Fix kernel panic in get_arm64_sve during core dump

The coredump logic calls get_arm64_sve twice: once to get the note size,
and once to get the data. The note size calculation depended on the
volatile PCB_FP_SVEVALID flag. If this flag was cleared between the
two calls (e.g., due to a context switch clearing the flag to comply
with the ABI), the second call would expect a smaller buffer size than
the first, triggering a KASSERT panic ("invalid size").

Fix this by ensuring the SVE state is saved to the PCB before we decide
whether to use SVE or VFP.

PR: 292195
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D54532