[relaxed-simd] Add experimental feature flag

This is not used anywhere yet, and will be later used to gate
instructions like FMA (which is prototyped).

Bug: v8:11583
Change-Id: I2eac60cde6998490fcf3aee11c618ab5478346ee
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2770605Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73519}

src/wasm/wasm-feature-flags.h

@@ -32,7 +32,12 @@
   /* Memory64 proposal. */                                                     \
   /* https://github.com/WebAssembly/memory64 */                                \
   /* V8 side owner: clemensb */                                                \
-  V(memory64, "memory64", false)
+  V(memory64, "memory64", false)                                               \
+                                                                               \
+  /* Relaxed SIMD proposal. */                                                 \
+  /* https://github.com/WebAssembly/relaxed-simd */                            \
+  /* V8 side owner: zhin */                                                    \
+  V(relaxed_simd, "relaxed simd", false)
 
 // #############################################################################
 // Staged features (disabled by default, but enabled via --wasm-staging (also