Fix build on i586 arch
Fix build on i586 arch

Case is not handled in signals.c and fallback leaves a variable unused,
which generates a warning, which generates an error.

Instead add support for i586 and mips, and provide an explicit warning
if architecture is not supported.

Resolves: LE-6496
Change-Id: Ic4ab7d66e3c3acc7eb4fb8676540fdbbf7c9e07c

legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af