* Replaces the exit assembly with the function from std.
* Reads the top-of-stack struct at program startup that can get information
like the pid.
* Changes the read and write functions to use the Pread and Pwrite syscalls
instead of the depreciated _READ and _WRITE
* Changes the openat function to use flags instead of perms.
Plan9 does not support perms when opening, just when creating.
* Adds an errstr function to read the errstr buf created by the kernel
Implements issue #6451.
This was needed to support allocation on Plan 9 and now other operating
systems like DOS can also use it.
It is a modified version of the WasmAllocator since wasm also uses a
sbrk-esque allocation system.
This commit also adds the necessary system bits for sbrk to work on plan 9.
Most of this migration was performed automatically with `zig fmt`. There
were a few exceptions which I had to manually fix:
* `@alignCast` and `@addrSpaceCast` cannot be automatically rewritten
* `@truncate`'s fixup is incorrect for vectors
* Test cases are not formatted, and their error locations change
