🐛 Add -fPIC to CFLAGS for Arch x86_64 (#2085)

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',
     ]