Add .uninit (NOLOAD) section to tb-app.x for defmt-rtt compatibility

[aq1018]

Problem

defmt-rtt places its buffer in .uninit.* sections and expects them to be NOLOAD (not included in the flash image). qingke-rt's link.x doesn't define a .uninit section, so the linker falls back to placing these symbols in a default loadable section with a RAM VMA (0x20000000+).

When wlink sees a loadable segment at a RAM address, it interprets the VMA as a flash offset and attempts to produce a binary spanning from 0x00000000 to 0x20000xxx. This results in a ~500MB output that fails to flash.

Fix

Define an explicit .uninit (NOLOAD) section in tb-app.x placed in RAM after .bss. The NOLOAD attribute tells the linker not to generate a load segment, so wlink only sees the actual flash content.

[aq1018]

For reference, cortex-m/cortex-m-rt has a NOLOAD section here: https://github.com/rust-embedded/cortex-m/blob/master/cortex-m-rt/link.x.in#L175-L183

[aq1018]

Related PR on qingke: ch32-rs/qingke#19.

We can revert this if/when qingke fixes this issue.