wasm linker fails to use bss section for undefined bytes

[andrewrk]

zig version: 0.14.0-dev.2790+d4fe4698d

Observe:

var foo: [1000]u8 = undefined;
pub fn main() void {
    foo[100] = 1;
}

zig build-exe test.zig -target wasm32-wasi -fno-llvm -fno-lld -OReleaseFast

Then look at test.wasm with wasm-objdump -x. There are pages and pages of aaaaaaa. This is OK for debug mode but for release modes, it should lower these as zeroes so that the binary can be smaller.

...
  - 10006a0: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 10006b0: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 10006c0: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 10006d0: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 10006e0: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 10006f0: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 1000700: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 1000710: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 1000720: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 1000730: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 1000740: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 1000750: aaaa aaaa aaaa aaaa aaaa aaaa aaaa aaaa  ................
  - 1000760: aaaa aa                                  ...

Another thing to look into: even for debug builds, emit these the same as zero data, and use runtime memory initialization to memset the undef bytes to aa.