scudo: Fix the build of wrappers_c_test.cpp on Android.

[pcc]

Summary

The Android headers don't provide a declaration of valloc or pvalloc, so we
need to declare them ourselves.

[merge_guards_bot]

Build result: pass - 60516 tests passed, 0 failed and 726 were skipped.

Log files: console-log.txt, CMakeCache.txt

compiler-rt/lib/scudo/standalone/tests/wrappers_c_test.cpp

 #include <unistd.h>
 
 extern "C" {
 void malloc_enable(void);
 void malloc_disable(void);
 int malloc_iterate(uintptr_t base, size_t size,
                    void (*callback)(uintptr_t base, size_t size, void *arg),
                    void *arg);
+void *valloc(size_t size);
+void *pvalloc(size_t size);
 }
 
 // Note that every C allocation function in the test binary will be fulfilled
 // by Scudo (this includes the gtest APIs, etc.), which is a test by itself.
 // But this might also lead to unexpected side-effects, since the allocation and
 // deallocation operations in the TEST functions will coexist with others (see
 // the EXPECT_DEATH comment below).