WASI: make initial linear memory and stack size target-configurable

[cpunion]

Problem

At current main (60c30f2ff), the WASI toolchain unconditionally adds:

-Wl,--initial-memory=67108864
-z stack-size=10485760

internal/crosscompile/crosscompile_test.go also freezes the 64 MiB value. This applies even though targets/wasip1.json declares a much smaller default-stack-size and even when the generated program does not need a large host stack.

This makes a minimal WASI command reserve 1,024 WebAssembly pages (64 MiB) at instantiation, hides the resource benefit of stackless/embedded runtimes, and gives target authors no supported way to select an appropriate memory budget.

Small reproducer

Build any successful WASI command and inspect its memory section:

llgo build -target=wasip1 -o hello.wasm ./hello.go
wasm-objdump -x hello.wasm | grep -A2 '^Memory'

The result contains:

Memory[1]:
 - memory[0] pages: initial=1024

The fixed values come directly from internal/crosscompile/crosscompile.go rather than the resolved target profile.

Suggested direction

• Add target/profile fields for initial (and optionally maximum) linear memory.
• Use the resolved target stack-size policy instead of the unconditional 10 MiB linker stack.
• Keep conservative defaults for the direct GOOS=wasip1 GOARCH=wasm path, but allow named targets to override them.
• Add a linker-flag unit test plus a tiny module layout test proving the configured page count.

This is independent of the coroutine implementation; it is in the shared main-branch WASI toolchain.