It requires less boilerplate code for newer ports.  The _Static_assert
checks from internal setjmp are moved to its own internal test since
setjmp.h is included early by multiple headers (to generate
rtld-sizes.sym).

The riscv jmp_buf-macros.h check is also redundant, it is already
done by riscv configure.ac.

Checked with a build for the affected architectures.