Remove our binary of ccache

Our binary was rather old, and for a variety of reasons we haven't kept
it updated. We've been running into a handful of reliability issues that
would have been fixed with an update, and a few reproducibility /
correctness issues that may or may not be fixed with newer versions.

For local no-change full rebuilds, ccache can still save ~35% of the
build time (but adds a few minutes to initially populate the cache). But
most local uses should be using incremental builds anyways, not clean
rebuilds. Or you're doing builds of different configurations, which
wouldn't be cache hits either, and would make your cache even larger.

At a large scale, we haven't seen a significant performance difference
between having ccache on or off. This may be different if you've got
very good build locality, or a very large cache -- but if you've got
good build locality, it's reasonable to do incremental builds (not for
release builds, and while running `m installclean` in between builds).

So for our cases, we'd prefer the stability and correctness of not using
ccache, but if you still want to use ccache, continue setting USE_CCACHE
and also set CCACHE_EXEC to the path of your ccache executable.

Bug: 32748498
Bug: 72408185
Test: performance testing of USE_CCACHE=false vs true locally
Test: turned off ccache for a collection of targets
Test: CCACHE_EXEC=/usr/bin/ccache USE_CCACHE=true m
Change-Id: I7117fe3107bd98521051ae343038a38f7e855502

core/ccache.mk

@@ -14,6 +14,21 @@
 # limitations under the License.
 #
 
+# We no longer provide a ccache prebuilt.
+#
+# Ours was old, and had a number of issues that triggered non-reproducible
+# results and other failures. Newer ccache versions may fix some of those
+# issues, but at the large scale of our build servers, we weren't seeing
+# significant performance gains from using ccache -- you end up needing very
+# good locality and/or very large caches if you're building many different
+# configurations.
+#
+# Local no-change full rebuilds were showing better results, but why not just
+# use incremental builds at that point?
+#
+# So if you still want to use ccache, continue setting USE_CCACHE, but also set
+# the CCACHE_EXEC environment variable to the path to your ccache executable.
+ifneq ($(CCACHE_EXEC),)
 ifneq ($(filter-out false,$(USE_CCACHE)),)
   # The default check uses size and modification time, causing false misses
   # since the mtime depends when the repo was checked out
@@ -36,17 +51,11 @@ ifneq ($(filter-out false,$(USE_CCACHE)),)
   # See http://petereisentraut.blogspot.com/2011/09/ccache-and-clang-part-2.html
   export CCACHE_CPP2 := true
 
-  CCACHE_HOST_TAG := $(HOST_PREBUILT_TAG)
-  ccache := prebuilts/misc/$(CCACHE_HOST_TAG)/ccache/ccache
-  # Check that the executable is here.
-  ccache := $(strip $(wildcard $(ccache)))
-  ifdef ccache
   ifndef CC_WRAPPER
-      CC_WRAPPER := $(ccache)
+    CC_WRAPPER := $(CCACHE_EXEC)
   endif
   ifndef CXX_WRAPPER
-      CXX_WRAPPER := $(ccache)
-    endif
-    ccache =
+    CXX_WRAPPER := $(CCACHE_EXEC)
+  endif
 endif
 endif