eco32: head: save $1 before clobbering it

$1 can't in fact be used without worries, it might be holding
the address for an offending load/store, so it needs to be
saved before it can be used.
And since we load the current_pgd from a global variable,
gas will use $1 to get the full 32-bit address to that,
so we need to save it.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>