Fix regression for _setjmp, fucntion does not use stack and adjusting
it causes restored stack to be incorrect.

Change offsets to (N * 8) to reduce chance of register clobber and mistakes.
Simplify integer loading, use 'li <dest>, <value>' instead of x0/zero register
Adjust _JB_SIGMASK to not collide with saved registers.

riscv64 libc setjmp functions,
Based on arm64 versions
this implementation is missing jmpxor security enhancement.
Good enough deraadt@

Change offsets to (N * 8) to reduce chance of register clobber and mistakes.
Simplify integer loading, use 'li <dest>, <value>' instead of x0/zero register
Adjust _JB_SIGMASK to not collide with saved registers.

riscv64 libc setjmp functions,
Based on arm64 versions
this implementation is missing jmpxor security enhancement.
Good enough deraadt@

riscv64 libc setjmp functions,
Based on arm64 versions
this implementation is missing jmpxor security enhancement.
Good enough deraadt@