tccgen: fix casts using code from upstream

We detected issues: tcc-mob built with us was failing to compile code
like:

    unsigned long x = 0xcafecafe;

It was applying sign extension, producing a `x == 0xffffffffcafecafe`.
Instead, when built with GCC tcc-mob produced the proper
`x == 0xcafecafe`.  This is because our casts are faulty in RISC-V
because it always sign-extends and we need to clear those high bits
after the sign-extension.

Proper compilation should produce this after the value is loaded:

    slli reg, reg, 0x20 // shift left 32 times
    srli reg, reg, 0x20 // and back

Meaning the highest 32 bits are cleared.

Our casts didn't detect the need for this, and many appearances of
integer constants in tcc-mob were miscompiled, producing future errors
in the chain: `load`, code generation, elf...

This commit backports the `gen_cast` function from tcc-mob and
manipulates the code around it to make it fit in our internals.  It's not
superclean, but it should work.

* arm64-gen.c (gen_cvt_csti): New function for char to int conversion.
* i386-gen.c (gen_cvt_csti): Likewise.
* x86_64-gen.c (gen_cvt_csti): Likewise.
(gen_cvt_sxtw): New function for word sign-extension.
* tcc.h: Add them to headers.
* tccgen.c (gen_cast): Update from upstream.
(btype_size): Add function.