x86: Use separate variable for TLSDESC XSAVE/XSAVEC state size (bug 32810)

x86: Use separate variable for TLSDESC XSAVE/XSAVEC state size (bug 32810)

Previously, the initialization code reused the xsave_state_full_size
member of struct cpu_features for the TLSDESC state size.  However,
the tunable processing code assumes that this member has the
original XSAVE (non-compact) state size, so that it can use its
value if XSAVEC is disabled via tunable.

This change uses a separate variable and not a struct member because
the value is only needed in ld.so and the static libc, but not in
libc.so.  As a result, struct cpu_features layout does not change,
helping a future backport of this change.

Fixes commit 9b7091415af47082664717210ac49d51551456ab ("x86-64:
Update _dl_tlsdesc_dynamic to preserve AMX registers").

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>