ash: less hackish implementation of evaltreenr()

Defining a function alias with __attribute__ ((alias("evaltree"),__noreturn__))
is not that usual, and clang had a bug which made it misunderstand
this construct.

Switch to:
ALWAYS_INLINE NORETURN evaltreenr() { evaltree(); unreachable(); }

Older gcc's do not know unreachable(), on them we pay the price of having
a few extra calls to abort():

function                                             old     new   delta
evalsubshell                                         151     156      +5
evalpipe                                             357     362      +5
argstr                                              1141    1144      +3

On newer gcc, code size does not change.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/platform.h

@@ -45,6 +45,13 @@
 
 #define UNUSED_PARAM __attribute__ ((__unused__))
 #define NORETURN __attribute__ ((__noreturn__))
+
+#if __GNUC_PREREQ(4,5)
+# define bb_unreachable(altcode) __builtin_unreachable()
+#else
+# define bb_unreachable(altcode) altcode
+#endif
+
 /* "The malloc attribute is used to tell the compiler that a function
  * may be treated as if any non-NULL pointer it returns cannot alias
  * any other pointer valid when the function returns. This will often