[MC][ELF] Mark Solaris objects as ELFOSABI_SOLARIS

Prompted by D107747 <https://reviews.llvm.org/D107747>, it seems prudent to
mark objects as `ELFOSABI_SOLARIS` on Solaris.

Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D107748

llvm/include/llvm/MC/MCELFObjectWriter.h

@@ -78,6 +78,8 @@ public:
       case Triple::PS4:
       case Triple::FreeBSD:
         return ELF::ELFOSABI_FREEBSD;
+      case Triple::Solaris:
+        return ELF::ELFOSABI_SOLARIS;
       default:
         return ELF::ELFOSABI_NONE;
     }

llvm/test/MC/ELF/osabi-solaris.s

@@ -0,0 +1,2 @@
+# RUN: llvm-mc -filetype obj -triple amd64-solaris %s | llvm-readobj -hS - | FileCheck %s
+# CHECK: OS/ABI: Solaris