tcg-sparc: Pull half the slow path out to trampolines

Not essential for 32-bit, but significant for 64-bit PIE.

We need the call to the trampoline to load an address against
which we can apply a small displacement, rather than building
a full 64-bit address.

But we do get a small code size decrease for 32-bit as well.

Signed-off-by: Richard Henderson <rth@twiddle.net>