[Ada] Use uniform EH setting on ARM/Linux platforms

[Eric Botcazou]

As pointed out by Joseph.  Applied on the mainline.


2018-09-13  Eric Botcazou  <ebotca...@adacore.com>

        * Makefile.rtl (arm% linux-gnueabi%): Always set EH_MECHANISM to -arm.

-- 
Eric Botcazou