build: fix .tls_template_size calculation

For an unknown reason, the current calculation of .tls_template_size
yields 0x10 instead of the correct value.  This results in part of the
initial tls block being freed by arch::setup(), and subsequent
corruption.

Fix by switching to the ld SIZEOF() operator.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>