NVIDIA PTX backend: emit string literal globals as .const arrays

[Zaneham]

PTX backend doesn't lower string literals yet. E110 refusal fires on any device-side "..." until this lands.

Easiest of the three. For each byte-init global, emit .const .align 1 .b8 .strN[len] = { 'h', 'e', ... }; at module scope, and lower BIR_GLOBAL_REF to mov.u64 %rd, .strN. The existing PTX BIR_GLOBAL_REF handler is a no-op stub today, so it's basically a new code path rather than touching anything.

Bytes live at M->strings[c->d.bytes.off], length c->d.bytes.len. bir_global_is_bytes(M, gi) tells you which globals.

[Zaneham]

Phase 2 mention is cause I've wired up sysprint which was the first part which comes with several fixes