[archs] Add -fPIC to CFLAGS for Arch x86_64

Since, at this point, we know that we need to add the mentioned flags at
some point, let's be brave and try to add it at Arch level (since we
need `-fPIC` for `ArchARMv7_a` it may be also the case for `Archx86_64`)

See also: #2073 (comment)

Author: opacam

pythonforandroid/archs.py

@@ -289,6 +289,7 @@ class Archx86_64(Arch):
         '-mpopcnt',
         '-m64',
         '-mtune=intel',
+        '-fPIC',
     ]