Fix asm format of bn_mul.h for more portability

Found by Barry K. Nathan.

Quoting from http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html:

"You can put multiple assembler instructions together in a single asm
template, separated by the characters normally used in assembly code for the
system. A combination that works in most places is a newline to break the
line, plus a tab character to move to the instruction field (written as
‘\n\t’). Sometimes semicolons can be used, if the assembler allows semicolons
as a line-breaking character. Note that some assembler dialects use semicolons
to start a comment."

ChangeLog

@@ -47,6 +47,8 @@ Bugfix
      CA for use as an end entity certificate. (This had been removed in
      1.3.6.)
    * Fix preprocessor checks for bn_mul PPC asm (found by Barry K. Nathan).
+   * Use \n\t rather than semicolons for bn_mul asm, since some assemblers
+     interpret semicolons as comment delimiters (found by Barry K. Nathan).
 
 = PolarSSL 1.3.7 released on 2014-05-02
 Features