rustc: Tweak exe allocator for MinGW/rumprun

Both of these targets have jemalloc disabled unconditionally right now, so using
`maybe_jemalloc` here isn't right. This fixes the case where a Linux compiler
(which is itself configured to use jemalloc) attempts to cross-compile to MinGW,
causing it to try to find an `alloc_jemalloc` crate (and failing).

src/librustc_back/target/windows_base.rs

@@ -84,7 +84,6 @@ pub fn opts() -> TargetOptions {
             "rsend.o".to_string()
         ),
         custom_unwind_resume: true,
-        exe_allocation_crate: super::maybe_jemalloc(),
 
         .. Default::default()
     }

src/librustc_back/target/x86_64_rumprun_netbsd.rs

@@ -21,6 +21,7 @@ pub fn target() -> Target {
     base.position_independent_executables = false;
     base.disable_redzone = true;
     base.no_default_libraries = false;
+    base.exe_allocation_crate = "alloc_system".to_string();
 
     Target {
         llvm_target: "x86_64-rumprun-netbsd".to_string(),