Temporarily disable tcmalloc due to compiler-explorer crash (#4177)

Per @axsaucedo on #4176, tcmalloc expects cpu information that
compiler-explorer is lacking in its sandboxing. There's probably a
better fix to be had, this is intended to be temporary.

.bazelrc

@@ -109,7 +109,9 @@ build --enable_platform_specific_config
 build:linux --define=pfm=1
 
 # Enable TCMalloc on Linux as well.
-build:linux --custom_malloc=@tcmalloc//tcmalloc
+# TODO: tcmalloc needs to support compiler-explorer; see:
+# https://github.com/carbon-language/carbon-lang/issues/4176
+# build:linux --custom_malloc=@tcmalloc//tcmalloc
 
 # Allow users to override any of the flags desired by importing a user-specific
 # RC file here if present.