Fix malloc_utils features (sysmalloc) (#7770)

Follow-up to:

- https://github.com/sigp/lighthouse/pull/7764

The `heaptrack` feature added in my previous PR was ineffective, because the jemalloc feature was turned on by the Linux target-specific dependency.

This PR tweaks the features such that:

- The jemalloc feature is just used to control whether jemalloc is compiled in. It is enabled on Linux by the target-specific dependency (see `lighthouse/Cargo.toml`), and completely disabled on Windows.
- If the `sysmalloc` feature is set on Linux then it overrides jemalloc when selecting an allocator, _even if_ the jemalloc feature is enabled (and the jemalloc dep was compiled).

common/malloc_utils/Cargo.toml

@@ -4,10 +4,23 @@ version = "0.1.0"
 authors = ["Paul Hauner <paul@paulhauner.com>"]
 edition = { workspace = true }
 
+# Features are not rich enough to express the complexity of our defaults, so we choose to just
+# use the jemalloc feature to control whether the dependency is compiled, but avoid using it if
+# the `sysmalloc` feature is set.
+#
+# On Windows, setting the jemalloc feature will result in a compile-time error.
 [features]
+default = []
 mallinfo2 = []
-jemalloc = ["tikv-jemallocator", "tikv-jemalloc-ctl"]
+# The jemalloc feature enables the compilation of jemalloc dependencies. Jemalloc is also the
+# default allocator, unless `sysmalloc` is set.
+#
+# It should be turned off on Windows.
+jemalloc = ["tikv-jemalloc-ctl", "tikv-jemallocator"]
 jemalloc-profiling = ["tikv-jemallocator/profiling"]
+# Force the use of system malloc (or glibc) rather than jemalloc.
+# This is a no-op on Windows where jemalloc is always disabled.
+sysmalloc = []
 
 [dependencies]
 libc = "0.2.79"