Convert asm keyword into __asm__.

Neither the asm() nor the __asm__() keyword is part of the C99
standard, but while GCC accepts the former in C89 syntax, it is not
accepted in C99 unless GNU extensions are turned on (with -fasm). The
latter form is accepted in any syntax as an extension (without
requiring further command-line options).

Sun Studio C99 compiler also does not accept asm() while accepting
__asm__(), albeit reporting warnings that it's not valid C99 syntax.

Originally committed as revision 15627 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/optimization.txt

@@ -154,17 +154,17 @@ The minimum guaranteed alignment is written in the .h files, for example:
 General Tips:
 -------------
 Use asm loops like:
-asm(
+__asm__(
     "1: ....
     ...
     "jump_instruciton ....
 Do not use C loops:
 do{
-    asm(
+    __asm__(
         ...
 }while()
 
-Use asm() instead of intrinsics. The latter requires a good optimizing compiler
+Use __asm__() instead of intrinsics. The latter requires a good optimizing compiler
 which gcc is not.