pw_malloc: Add alwayslink

The wrapped dynamic memory operations may be used exclusively in source
files coming from the toolchain (not in any sources in project code
itself). In this case, these symbols will appear unused and be garbage
collected by the linker.

I'm not entirely sure if this is the correct fix. (What if your binary
really does not use dynamic memory allocation, will cc_binary still
burden it with a dep on malloc when you set
--custom_malloc=@pigweed//pw_malloc?)

Bug: 342510882
Change-Id: Iaffa27dc5614d27d3a81ac869e96fbf4dba57689
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/256453
Lint: Lint 🤖 <[email protected]>
Presubmit-Verified: CQ Bot Account <[email protected]>
Reviewed-by: Alexei Frolov <[email protected]>
Commit-Queue: Ted Pudlik <[email protected]>
Docs-Not-Needed: Ted Pudlik <[email protected]>

pw_malloc/BUILD.bazel

@@ -73,6 +73,7 @@ pw_facade(
         ":common",
         ":config_override",
     ],
+    alwayslink = 1,
 )
 
 label_flag(