[MC] set OpenBSD's ELFOSABI by default (llvm#98158) (llvm#98553)

This matches what is done for FreeBSD.

OpenBSD has a few special program header types, and other such ELF
extensions. Setting the ELFOSABI like so will allow LLD to support them
without needlessly impacting non-OpenBSD ELFs.

Testing strategy matches 06cecdc.

Take two of llvm#98158 / b64c1de, which was
reverted in llvm#98494 / c026135.
Preexisting test is fixed now.

Author: Ericson2314

lld/test/ELF/basic-sparcv9.s

@@ -17,7 +17,7 @@ _start:
 # CHECK-NEXT:     Class: 64-bit (0x2)
 # CHECK-NEXT:     DataEncoding: BigEndian (0x2)
 # CHECK-NEXT:     FileVersion: 1
-# CHECK-NEXT:     OS/ABI: SystemV (0x0)
+# CHECK-NEXT:     OS/ABI: OpenBSD (0x0)
 # CHECK-NEXT:     ABIVersion: 0
 # CHECK-NEXT:     Unused: (00 00 00 00 00 00 00)
 # CHECK-NEXT:   }

llvm/include/llvm/MC/MCELFObjectWriter.h

@@ -78,6 +78,8 @@ class MCELFObjectTargetWriter : public MCObjectTargetWriter {
         return ELF::ELFOSABI_FREEBSD;
       case Triple::Solaris:
         return ELF::ELFOSABI_SOLARIS;
+      case Triple::OpenBSD:
+        return ELF::ELFOSABI_OPENBSD;
       default:
         return ELF::ELFOSABI_NONE;
     }

llvm/test/MC/ELF/osabi.s

@@ -10,3 +10,7 @@
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-freebsd %s | llvm-readobj -h - | \
 # RUN:   FileCheck %s --check-prefix=FREEBSD
 # FREEBSD: OS/ABI: FreeBSD
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-openbsd %s | llvm-readobj -h - | \
+# RUN:   FileCheck %s --check-prefix=OPENBSD
+# OPENBSD: OS/ABI: OpenBSD