Disable DWARF in linker options for i686-unknown-uefi

This fixes an lld warning:
> warning: linker stderr: rust-lld: section name .debug_frame is longer
> than 8 characters and will use a non-standard string table

See https://reviews.llvm.org/D69594 for details of where the warning was
added.

This warning only occurs with the i686 UEFI target, not x86_64 or
aarch64. The x86_64 target uses an LLVM target of
`x86_64-unknown-windows` and aarch64 uses `aarch64-unknown-windows`, but
i686 uses `i686-unknown-windows-gnu` (note the `-gnu`). See comments in
`i686_unknown_uefi.rs` for details of why.

The `.debug_frame` section should not actually be needed; UEFI targets
provide a separate PDB file for debugging. Disable DWARF (and by
extension the `.debug_frame` section) by passing `/DEBUG:NODWARF` to lld.

Tested with:
export RUSTC_LOG=rustc_codegen_ssa::back::link=info
cargo +stage1 build --release --target i686-unknown-uefi

compiler/rustc_target/src/spec/targets/i686_unknown_uefi.rs

@@ -5,7 +5,7 @@
 // The cdecl ABI is used. It differs from the stdcall or fastcall ABI.
 // "i686-unknown-windows" is used to get the minimal subset of windows-specific features.
 
-use crate::spec::{RustcAbi, Target, base};
+use crate::spec::{LinkerFlavor, Lld, RustcAbi, Target, add_link_args, base};
 
 pub(crate) fn target() -> Target {
     let mut base = base::uefi_msvc::opts();
@@ -24,6 +24,13 @@ pub(crate) fn target() -> Target {
     base.features = "-mmx,-sse,+soft-float".into();
     base.rustc_abi = Some(RustcAbi::X86Softfloat);
 
+    // Turn off DWARF. This fixes an lld warning, "section name .debug_frame is longer than 8
+    // characters and will use a non-standard string table". That section will not be created if
+    // DWARF is disabled.
+    //
+    // This is only needed in the i686 target due to using the `-gnu` LLVM target (see below).
+    add_link_args(&mut base.post_link_args, LinkerFlavor::Msvc(Lld::No), &["/DEBUG:NODWARF"]);
+
     // Use -GNU here, because of the reason below:
     // Background and Problem:
     //   If we use i686-unknown-windows, the LLVM IA32 MSVC generates compiler intrinsic