Fix definition of _setjmp

The #pragma weak does not work, the symbol is not defined and when
already defined, it is not made weak. Moreover, it does not make the
symbol global so we need to declare it with a FUNCTION.

If we want the symbol to be weak, then we should use .weak to do so. The
_longjmp symbol should also be changed to match, in that case.

Fixes #12912.

ASM code fixes for PIC compilation.

Fixed the x86_64 setjmp implementation.

* typedef for jmp_buf was using int where it should be long.
* setjmp was clearing the buffer pointer rather than the signal mask before
calling sigsetjmp.
* KDL now works without crashing on x86_64.

Implementations of some libroot functions for x86_64.

These are the functions required by the kernel. These are all full
implementations except for system_time(), which will be implemented later.

ASM code fixes for PIC compilation.

Fixed the x86_64 setjmp implementation.

* typedef for jmp_buf was using int where it should be long.
* setjmp was clearing the buffer pointer rather than the signal mask before
calling sigsetjmp.
* KDL now works without crashing on x86_64.

Implementations of some libroot functions for x86_64.

These are the functions required by the kernel. These are all full
implementations except for system_time(), which will be implemented later.