riscv: Implement long double support

riscv: Implement long double support

for the implementation of operations we can reuse the ones
from lib/lib-arm64.c, risc-v long double is also float128.
Also implement ggoto, and PDIV, and use t0 in load/store as
temporary register if necessary, not one given by get_reg
(the latter can destroy assignments of long double parameters
in function calls that are already set up).

This let's us compile tcc.c and tcctest.c, though both
don't yet work.