rust: Use 'pie' relocation mode under UML

UML expects a position independent executable for some reason, so tell
rustc to generate pie objects. Otherwise we get a bunch of relocations
we can't deal with in libcore.

Signed-off-by: David Gow <davidgow@google.com>

Author: sulix

Makefile

@@ -570,7 +570,6 @@ KBUILD_RUSTFLAGS := $(rust_common_flags) \
 		    -Cpanic=abort -Cembed-bitcode=n -Clto=n \
 		    -Cforce-unwind-tables=n -Ccodegen-units=1 \
 		    -Csymbol-mangling-version=v0 \
-		    -Crelocation-model=static \
 		    -Zfunction-sections=n \
 		    -Dclippy::float_arithmetic
 

scripts/generate_rust_target.rs

@@ -228,5 +228,11 @@ fn main() {
         ts.push("target-endian", "big");
     }
 
+    if cfg.has("UML") {
+        ts.push("relocation-model", "pie");
+    } else {
+        ts.push("relocation-model", "static");
+    }
+
     println!("{}", ts);
 }