fix build on: define own version of ALLOCM_LG_ALIGN

Summary:
On --platform=gcc-4.6.2-glibc-2.13-fb the build would fail because

fbcode/third-party/gcc-4.6.2-glibc-2.13-fb/libgcc/libgcc-4.6.2/b4e37bb/include/c++/4.6.2/bits/basic_fbstring_malloc.h

defines it's own version of ALLOCM_SUCCESS & co., but not ALLOCM_LG_ALIGN :(

The '#ifdef ALLOCM_SUCCESS' check in 'folly/Malloc.h' would pass even
though we didn't include jemalloc.

Test Plan: build it

Reviewed By: philipp@fb.com

FB internal diff: D562273

folly/Malloc.h

@@ -84,7 +84,6 @@ __attribute__((weak));
 
 #define ALLOCM_ZERO    64
 #define ALLOCM_NO_MOVE 128
-#define ALLOCM_LG_ALIGN(la) (la)
 #endif
 
 #ifdef _LIBSTDCXX_FBSTRING