[Ada] Remove -mthreads from Linker_Options pragma for x86-lynx178e

[Pierre-Marie de Rodat]

The -mthreads flag is no longer needed.  It was deprecated in 2.2.4 and
in 2.2.5 causes the linker to reference a non-existent directory.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

        * libgnarl/s-osinte__lynxos178e.ads: Remove -mthreads switch.
diff --git a/gcc/ada/libgnarl/s-osinte__lynxos178e.ads b/gcc/ada/libgnarl/s-osinte__lynxos178e.ads
--- a/gcc/ada/libgnarl/s-osinte__lynxos178e.ads
+++ b/gcc/ada/libgnarl/s-osinte__lynxos178e.ads
@@ -47,10 +47,6 @@ with System.Multiprocessors;
 package System.OS_Interface is
    pragma Preelaborate;
 
-   pragma Linker_Options ("-mthreads");
-   --  Selects the POSIX 1.c runtime, rather than the non-threading runtime or
-   --  the deprecated legacy threads library.
-
    subtype int            is Interfaces.C.int;
    subtype short          is Interfaces.C.short;
    subtype long           is Interfaces.C.long;