Add a build flag for aarch64 Neoverse 512TVB optimizations and gate behind CPU detection

`lore-base/build.rs` passed `-mcpu=neoverse-512tvb` unconditionally to the C compiler for all `linux/aarch64` targets, causing binaries to crash with `illegal hardware instruction` on hardware that does not implement SVE2 and other Neoverse 512TVB extensions. Introduce a `neoverse-512tvb` Cargo feature to opt in to the optimization, and gate it behind a `/proc/cpuinfo` SVE2 check so the flag is only applied when the hardware supports it.

Fixes #42

Signed-off-by: subhramit <subhramit.bb@live.in>

Author: subhramit

lore-base/build.rs

@@ -26,7 +26,20 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         .includes(Some(native_dir.join("thirdparty")));
 
     if platform == "linux" && arch == "aarch64" {
-        cc_builder.flag("-mcpu=neoverse-512tvb");
+        if env::var("LORE_CPU_NEOVERSE_512TVB").is_ok() {
+            let cpuinfo = std::fs::read_to_string("/proc/cpuinfo").unwrap_or_default();
+            if cpuinfo.contains("sve2") {
+                cc_builder.flag("-mcpu=neoverse-512tvb");
+            } else {
+                println!(
+                    "cargo:warning=LORE_CPU_NEOVERSE_512TVB is set but SVE2 not detected in /proc/cpuinfo; binary may crash with illegal hardware instruction. Unset LORE_CPU_NEOVERSE_512TVB to build for generic aarch64"
+                );
+            }
+        } else {
+            println!(
+                "cargo:warning=Building rpmalloc without -mcpu=neoverse-512tvb; binary may be slower on Graviton3+. Set LORE_CPU_NEOVERSE_512TVB=1 to opt in"
+            );
+        }
     }
 
     if cc_builder.get_compiler().is_like_msvc() {