jemalloc_helper: Limit the mm_malloc.h hack to glibc on linux (#8425)

Summary:
Original author: kraj (#8413)

We have a hack to ensure clang's `posix_memalign()` hack works to be
compatible with glibc's `posix_memalign()` declaration. Our side of the
hack is irrelevant and should be omitted when not using glibc.

Pull Request resolved: #8425

Reviewed By: mrambacher

Differential Revision: D29239029

Pulled By: ajkr

fbshipit-source-id: 12b900f50a4823b880a6558f25d8590dbfc0aa26

port/jemalloc_helper.h

@@ -5,7 +5,7 @@
 
 #pragma once
 
-#if defined(__clang__)
+#if defined(__clang__) && defined(__GLIBC__)
 // glibc's `posix_memalign()` declaration specifies `throw()` while clang's
 // declaration does not. There is a hack in clang to make its re-declaration
 // compatible with glibc's if they are declared consecutively. That hack breaks