Avoid zero-initializing our most-used buffers.

The StringPrintf one is heavily used and brings the overhead versus
fmtlib down to 1.5x rather than 2x. I don't have a convenient benchmark
for the other two.

Test: libbase tests & benchmarks
Bug: http://b/155324241
Change-Id: I9e704a360846d5520c53f668e7c315b0c0ea55f8
(cherry picked from commit 8c253d4d42018d77b78de8f8c70c6204f5aa6d0e)
diff --git a/base/stringprintf.cpp b/base/stringprintf.cpp
index 78e1e8d..e83ab13 100644
--- a/base/stringprintf.cpp
+++ b/base/stringprintf.cpp
@@ -25,7 +25,7 @@
 
 void StringAppendV(std::string* dst, const char* format, va_list ap) {
   // First try with a small fixed size buffer
-  char space[1024];
+  char space[1024] __attribute__((__uninitialized__));
 
   // It's possible for methods that use a va_list to invalidate
   // the data in it upon use.  The fix is to make a copy