ash: less hackish implementation of evaltreenr()

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>