WasmPageAllocator: fix bug not aligning allocations

[marler8997]

Implements aligned allocations in WasmPageAllocator.

My changes to arena allocator (#5736) exposed a bug in WasmPageAllocator when allocating a block aligned to 262144 bytes. Note that this appears to be an existing bug unrelated to the new allocator changes, but exposed by them. WasmPageAllocator was not aligning allocations, but its page size is 64K so the bug is unlikely to be triggered unless you are doing very highly aligned allocations.

Here is the stacktrace of the bug:

327/1431 heap.test "std-wasm32-wasi-Debug-bare-single ArenaAllocator"...reached unreachable codeError: failed to run main module `/home/vsts/work/1/s/zig-cache/o/dnpZookktBQLFN5oOKiztk1CZcNp2WOzT3Ma5Gzm6ogPHQ9uvB0dGKFtpYMroB59/test.wasm`

Caused by:
    0: failed to invoke `_start`
    1: wasm trap: unreachable, source location: @4f82
       wasm backtrace:
         0: <unknown>!os.abort
         1: <unknown>!builtin.default_panic
         2: <unknown>!debug.assert
         3: <unknown>!mem.ValidationAllocator(*mem.Allocator).alloc
         4: <unknown>!mem.Allocator.callAllocFn
         5: <unknown>!mem.Allocator.allocAdvanced.4426
         6: <unknown>!mem.Allocator.alignedAlloc.3219
         7: <unknown>!heap.testAllocatorLargeAlignment
         8: <unknown>!heap.test "std-wasm32-wasi-Debug-bare-single ArenaAllocator"
         9: <unknown>!std.special.main
         10: <unknown>!_start
       

Tests failed. Use the following command to reproduce the failure: