build(core/emulator): build with -fPIE

Otherwise "cargo test" fails with errors like:

   Compiling trezor_lib v0.1.0 (embed/rust)
error: linking with `cc` failed: exit status: 1
  |
  = note: ld: build/unix/rust/debug/build/trezor_lib-a046f8e3db087595/out/libcore_lib.a(obj.o): relocation R_X86_64_32S against `.rodata.types.1' can not be used when making a PIE object; recompile with -fPIE

core/SConscript.unix

@@ -398,7 +398,7 @@ env.Replace(
     CCFLAGS='$COPT '
     '-g3 '
     '-std=gnu99 -Wall -Werror -Wuninitialized -Wno-missing-braces '
-    '-fdata-sections -ffunction-sections ' + CCFLAGS_MOD,
+    '-fdata-sections -ffunction-sections -fPIE ' + CCFLAGS_MOD,
     CCFLAGS_QSTR='-DNO_QSTR -DN_X64 -DN_X86 -DN_THUMB',
     LIBS=['m'],
     CPPPATH=[