Strip trailing whitespace in bn_mul.h

Remove the trailing whitespace from the inline assembly for AMD64 target, to
overcome a warning in Clang, which was objecting to the string literal
generated by the inline assembly being greater than 4096 characters specified
by the ISO C99 standard. (-Woverlength-strings)

This is a cosmetic change and doesn't change the logic of the code in any way.

This change only fixes the problem for AMD64 target, and leaves other targets as
they are.

Fixes Mbed-TLS#482.

include/mbedtls/bn_mul.h

@@ -170,19 +170,19 @@
 
 #define MULADDC_INIT                        \
     asm(                                    \
-        "xorq   %%r8, %%r8          \n\t"
+        "xorq   %%r8, %%r8\n"
 
 #define MULADDC_CORE                        \
-        "movq   (%%rsi), %%rax      \n\t"   \
-        "mulq   %%rbx               \n\t"   \
-        "addq   $8,      %%rsi      \n\t"   \
-        "addq   %%rcx,   %%rax      \n\t"   \
-        "movq   %%r8,    %%rcx      \n\t"   \
-        "adcq   $0,      %%rdx      \n\t"   \
-        "nop                        \n\t"   \
-        "addq   %%rax,   (%%rdi)    \n\t"   \
-        "adcq   %%rdx,   %%rcx      \n\t"   \
-        "addq   $8,      %%rdi      \n\t"
+        "movq   (%%rsi), %%rax\n"           \
+        "mulq   %%rbx\n"                    \
+        "addq   $8, %%rsi\n"                \
+        "addq   %%rcx, %%rax\n"             \
+        "movq   %%r8, %%rcx\n"              \
+        "adcq   $0, %%rdx\n"                \
+        "nop    \n"                         \
+        "addq   %%rax, (%%rdi)\n"           \
+        "adcq   %%rdx, %%rcx\n"             \
+        "addq   $8, %%rdi\n"
 
 #define MULADDC_STOP                        \
         : "+c" (c), "+D" (d), "+S" (s)      \