Backport #1949 into mbedtls-2.7

Signed-off-by: Jeffrey Martin <Jeffrey_Martin@rapid7.com>

ChangeLog

@@ -5,6 +5,10 @@ mbed TLS ChangeLog (Sorted per branch, date)
 Bugfix
    * Run the AD too long test only if MBEDTLS_CCM_ALT is not defined.
      Raised as a comment in #1996.
+   * Extend register cleanup needed on MIPS assembly optimizations. Adds hi, lo
+     registers hints to the compiler for usage with the embedded MIPS assembly.
+     Reported in #1722, fix suggested by @aurel32 and submitted
+     by Jeffrey Martin, #1949.
 
 = mbed TLS 2.7.9 branch released 2018-12-21
 

include/mbedtls/bn_mul.h

@@ -734,7 +734,7 @@
         "sw     $10, %2         \n\t"   \
         : "=m" (c), "=m" (d), "=m" (s)                      \
         : "m" (s), "m" (d), "m" (c), "m" (b)                \
-        : "$9", "$10", "$11", "$12", "$13", "$14", "$15"    \
+        : "$9", "$10", "$11", "$12", "$13", "$14", "$15", "lo", "hi" \
     );
 
 #endif /* MIPS */