gc: add -gc boehm_leak for leak detection (#9464)

cmd/v/help/build-c.txt

@@ -72,11 +72,12 @@ see also `v help build`.
       
    -gc <mode>
       Use and link an optional garbage collector. 
-      Only `-gc boehm` is supported currently. You need to install a
-      `libgc-dev` package first, or install it manually from source: 
+      Only `-gc boehm` and `-gc boehm_leak` are supported currently.
+      You need to install a `libgc-dev` package first, or install it manually
+      from source: 
           https://github.com/ivmai/bdwgc
           
-      Note, this option is complementary to -autofree. The Boehm garbage
+      Note, `-gc boehm` is complementary to -autofree. The Boehm garbage
       collector is conservative, and it may make your program significantly
       slower if it does many small allocations in a loop. This option
       is intended *mainly* for reducing the memory usage of programs, that
@@ -84,6 +85,11 @@ see also `v help build`.
       environments like small VPSes, and for which a few ms of garbage
       collection pauses from time to time *do not matter much*.
 
+      The option `-gc boehm_leak` is intended for leak detection in
+      manual memory management. The function `gc_check_leaks()`
+      can be called to get detection results. This function is a no-op
+      when `-gc boehm_leak` is not supplied.
+
 # Miscellaneous:
    -printfn <fn_name>
       Print the content of the generated C function named fn_name.