The --wasm-write-protect-code-memory flag previously enforced W^X, that
is the WebAssembly code space was either writable or executable, but
never both at the same time. With compilation in background threads
concurrent to execution in the main thread, this simple scheme is no
longer viable because the same memory page can indeed be written to and
executed at the same time. Hence, this flag is currently broken and
disabled and the code space is always writable AND executable.

As a first step towards more security, we at least want to
write-protect the code space (when not required writable by compilation
threads) but at the same time keep it always executable (because of
concurrent execution in the main thread). That is, we no longer switch
between RX and RW (W^X), but rather between RX and RWX
(write-protection only).

This CL starts to change from W^X (which was broken) to
write-protection only when enabling --wasm-write-protect-code-memory.
This is the first of two CLs, where the followup CL will fix the
feature, and this CL merely prepares and cleans up the code. In
particular, this CL changes the permissions from RW to RWX (due to
concurrent execution) and renames `WasmCodeAllocator::SetExecutable()`
to `WasmCodeAllocator::SetWritable()` (and similarly named callers) to
be consistent with that change. Since the code space is now always
executable, this CL also removes now unneeded calls to
`SetExecutable(true)` in tests.

R=clemensb@chromium.org
CC=​​jkummerow@chromium.org

Bug: v8:11663
Change-Id: I2065eed6770215892b81daefbddf74a349e783cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2835237Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Daniel Lehmann <dlehmann@google.com>
Cr-Commit-Position: refs/heads/master@{#74041}