[Aarch64] Fix warning in _sbrk

[Christophe Lyon]

Hi,

While building newlib for Aarch64, I noticed a warning in _sbrk. This
small patch fixes it the same way it is done in the Arm version.

OK?

Christophe
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commit 828bf40d0d822425cbfb0cd69dbb4e48cca66592
Author: Christophe Lyon <christophe.lyon@linaro.org>
Date:   Mon Oct 1 19:08:40 2018 +0000

    [Aarch64] Fix warning in _sbrk
    
    2018-10-01  Christophe Lyon  <christophe.lyon@linaro.org>
    
    	* libgloss/aarch64/syscalls.c (_sbrk): Cast "__heap_limit" to
    	"char *".

diff --git a/libgloss/aarch64/syscalls.c b/libgloss/aarch64/syscalls.c
index 8198d3e..e6dd4bd 100644
--- a/libgloss/aarch64/syscalls.c
+++ b/libgloss/aarch64/syscalls.c
@@ -642,7 +642,7 @@ _sbrk (int incr)
 
   if ((heap_end + incr > stack_ptr)
       /* Honour heap limit if it's valid.  */
-      || ((__heap_limit != 0xcafedead) && (heap_end + incr > __heap_limit)))
+      || ((__heap_limit != 0xcafedead) && (heap_end + incr > (char *)__heap_limit)))
     {
       /* Some of the libstdc++-v3 tests rely upon detecting
          out of memory errors, so do not abort here.  */