[FreeBSD] Fix target conditionals for FreeBSD aarch64 builds

Despite being declared in the header unistd.h, the function sbrk is not available on FreeBSD when using the aarch64 architecture because FreeBSD make the decision to drop support for this deprecated function when porting the system to ARM.

The configuration script incorrectly hard-codes the support for this function as being always available, which is not correct. We need to ensure that it is not used on FreeBSD aarch64.

Author: CodingCarpincho

include/llvm/Config/config.h

@@ -131,7 +131,11 @@
 /* #undef HAVE_LSEEK64 */
 
 /* Define to 1 if you have the `mallctl' function. */
+#if defined(__FreeBSD__)
+#define HAVE_MALLCTL 1
+#else
 /* #undef HAVE_MALLCTL */
+#endif
 
 /* Define to 1 if you have the `mallinfo' function. */
 /* #undef HAVE_MALLINFO */
@@ -204,7 +208,13 @@
 #define HAVE_REALPATH 1
 
 /* Define to 1 if you have the `sbrk' function. */
+/* This is hard-coded to be not present on FreeBSD arm64, because
+   that platform has dropped support for this function. */
+#if defined(__FreeBSD__) && defined(__aarch64__)
+#undef HAVE_SBRK
+#else
 #define HAVE_SBRK 1
+#endif
 
 /* Define to 1 if you have the `setenv' function. */
 #define HAVE_SETENV 1