uses tikv_jemallocator::Jemalloc as the global allocator (#20149)

https://github.com/solana-labs/solana/pull/16346
switched default allocator from jemalloc to system allocator, but that
has shown regressions in form of higher ram usage causing nodes go OOM:
https://discord.com/channels/428295358100013066/439194979856809985/890413193858539580

This commit sets jemalloc as the default allocator.

validator/src/main.rs

@@ -86,6 +86,13 @@ use {
     },
 };
 
+#[cfg(not(target_env = "msvc"))]
+use jemallocator::Jemalloc;
+
+#[cfg(not(target_env = "msvc"))]
+#[global_allocator]
+static GLOBAL: Jemalloc = Jemalloc;
+
 #[derive(Debug, PartialEq)]
 enum Operation {
     Initialize,