Fix e_flags for 32-bit MIPS targets in generated object file

In rust-lang#95604 the compiler started generating a temporary symbols.o which is added
to the linker invocation. This object file has an `e_flags` which may be invalid
for 32-bit MIPS targets. Even though symbols.o doesn't contain code, linking
    with [lld fails](https://github.com/llvm/llvm-project/blob/main/lld/ELF/Arch/MipsArchTree.cpp#L79) with
```
rust-lld: error: foo-cgu.0.rcgu.o: ABI 'o32' is incompatible with target ABI 'n64'
```
because it omits the ABI bits (EF_MIPS_ABI_O32) so lld assumes it's using the
N64 ABI. This breaks linking on nightly for the out-of-tree [psx
target](ayrtonm/psx-sdk-rs#9), the builtin
mipsel-sony-psp target (cc @overdrivenpotato) and any other 32-bit MIPS
target using lld.

This PR sets the ABI in `e_flags` to O32 since that's the only ABI for 32-bit
MIPS that LLVM supports. It also sets other `e_flags` bits based on the target.
I had to bump the object crate version since some of these constants were [added
recently](gimli-rs/object#433). I'm not sure if this
PR needs a test, but I can confirm that it fixes the linking issue on both
targets I mentioned.

Cargo.lock

@@ -2509,9 +2509,9 @@ dependencies = [
 
 [[package]]
 name = "object"
-version = "0.28.1"
+version = "0.28.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7ce8b38d41f9f3618fc23f908faae61510f8d8ce2d99cbe910641e8f1971f084"
+checksum = "e42c982f2d955fac81dd7e1d0e1426a7d702acd9c98d19ab01083a6a0328c424"
 dependencies = [
  "crc32fast",
  "flate2",
@@ -3617,7 +3617,7 @@ dependencies = [
  "itertools",
  "jobserver",
  "libc",
- "object 0.28.1",
+ "object 0.28.4",
  "pathdiff",
  "regex",
  "rustc_apfloat",
@@ -5207,7 +5207,7 @@ checksum = "dd95b4559c196987c8451b4e14d08a4c796c2844f9adf4d2a2dbc9b3142843be"
 dependencies = [
  "gimli 0.26.1",
  "hashbrown 0.11.2",
- "object 0.28.1",
+ "object 0.28.4",
  "tracing",
 ]
 

compiler/rustc_codegen_ssa/Cargo.toml

@@ -41,6 +41,6 @@ rustc_target = { path = "../rustc_target" }
 rustc_session = { path = "../rustc_session" }
 
 [dependencies.object]
-version = "0.28.0"
+version = "0.28.4"
 default-features = false
 features = ["read_core", "elf", "macho", "pe", "unaligned", "archive", "write"]

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -20,7 +20,7 @@ use rustc_metadata::EncodedMetadata;
 use rustc_session::cstore::MetadataLoader;
 use rustc_session::Session;
 use rustc_target::abi::Endian;
-use rustc_target::spec::Target;
+use rustc_target::spec::{RelocModel, Target};
 
 use crate::METADATA_FILENAME;
 
@@ -132,15 +132,23 @@ pub(crate) fn create_object_file(sess: &Session) -> Option<write::Object<'static
     let mut file = write::Object::new(binary_format, architecture, endianness);
     match architecture {
         Architecture::Mips => {
-            // copied from `mipsel-linux-gnu-gcc foo.c -c` and
-            // inspecting the resulting `e_flags` field.
-            let e_flags = elf::EF_MIPS_CPIC
-                | elf::EF_MIPS_PIC
-                | if sess.target.options.cpu.contains("r6") {
-                    elf::EF_MIPS_ARCH_32R6 | elf::EF_MIPS_NAN2008
-                } else {
-                    elf::EF_MIPS_ARCH_32R2
-                };
+            let arch = match sess.target.options.cpu.as_ref() {
+                "mips1" => elf::EF_MIPS_ARCH_1,
+                "mips2" => elf::EF_MIPS_ARCH_2,
+                "mips3" => elf::EF_MIPS_ARCH_3,
+                "mips4" => elf::EF_MIPS_ARCH_4,
+                "mips5" => elf::EF_MIPS_ARCH_5,
+                s if s.contains("r6") => elf::EF_MIPS_ARCH_32R6,
+                _ => elf::EF_MIPS_ARCH_32R2,
+            };
+            // The only ABI LLVM supports for 32-bit MIPS CPUs is o32.
+            let mut e_flags = elf::EF_MIPS_CPIC | elf::EF_MIPS_ABI_O32 | arch;
+            if sess.target.options.relocation_model != RelocModel::Static {
+                e_flags |= elf::EF_MIPS_PIC;
+            }
+            if sess.target.options.cpu.contains("r6") {
+                e_flags |= elf::EF_MIPS_NAN2008;
+            }
             file.flags = FileFlags::Elf { e_flags };
         }
         Architecture::Mips64 => {