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

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `premerge-monolithic-linux` running on `premerge-linux-1` while building `llvm` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/153/builds/2651

Here is the relevant piece of the build log for the reference:
```
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'lld :: ELF/basic-sparcv9.s' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 2: /build/buildbot/premerge-monolithic-linux/build/bin/llvm-mc -filetype=obj -triple=sparc64-unknown-openbsd /build/buildbot/premerge-monolithic-linux/llvm-project/lld/test/ELF/basic-sparcv9.s -o /build/buildbot/premerge-monolithic-linux/build/tools/lld/test/ELF/Output/basic-sparcv9.s.tmp
+ /build/buildbot/premerge-monolithic-linux/build/bin/llvm-mc -filetype=obj -triple=sparc64-unknown-openbsd /build/buildbot/premerge-monolithic-linux/llvm-project/lld/test/ELF/basic-sparcv9.s -o /build/buildbot/premerge-monolithic-linux/build/tools/lld/test/ELF/Output/basic-sparcv9.s.tmp
RUN: at line 3: /build/buildbot/premerge-monolithic-linux/build/bin/ld.lld /build/buildbot/premerge-monolithic-linux/build/tools/lld/test/ELF/Output/basic-sparcv9.s.tmp -o /build/buildbot/premerge-monolithic-linux/build/tools/lld/test/ELF/Output/basic-sparcv9.s.tmp2
+ /build/buildbot/premerge-monolithic-linux/build/bin/ld.lld /build/buildbot/premerge-monolithic-linux/build/tools/lld/test/ELF/Output/basic-sparcv9.s.tmp -o /build/buildbot/premerge-monolithic-linux/build/tools/lld/test/ELF/Output/basic-sparcv9.s.tmp2
RUN: at line 4: /build/buildbot/premerge-monolithic-linux/build/bin/llvm-readobj --file-headers --sections -l --symbols /build/buildbot/premerge-monolithic-linux/build/tools/lld/test/ELF/Output/basic-sparcv9.s.tmp2    | /build/buildbot/premerge-monolithic-linux/build/bin/FileCheck /build/buildbot/premerge-monolithic-linux/llvm-project/lld/test/ELF/basic-sparcv9.s
+ /build/buildbot/premerge-monolithic-linux/build/bin/FileCheck /build/buildbot/premerge-monolithic-linux/llvm-project/lld/test/ELF/basic-sparcv9.s
+ /build/buildbot/premerge-monolithic-linux/build/bin/llvm-readobj --file-headers --sections -l --symbols /build/buildbot/premerge-monolithic-linux/build/tools/lld/test/ELF/Output/basic-sparcv9.s.tmp2
/build/buildbot/premerge-monolithic-linux/llvm-project/lld/test/ELF/basic-sparcv9.s:20:15: error: CHECK-NEXT: expected string not found in input
# CHECK-NEXT: OS/ABI: SystemV (0x0)
              ^
<stdin>:12:16: note: scanning from here
 FileVersion: 1
               ^
<stdin>:13:2: note: possible intended match here
 OS/ABI: OpenBSD (0xC)
 ^

Input file: <stdin>
Check file: /build/buildbot/premerge-monolithic-linux/llvm-project/lld/test/ELF/basic-sparcv9.s

-dump-input=help explains the following input dump.

Input was:
<<<<<<
           .
           .
           .
           7: ElfHeader { 
           8:  Ident { 
           9:  Magic: (7F 45 4C 46) 
          10:  Class: 64-bit (0x2) 
          11:  DataEncoding: BigEndian (0x2) 
          12:  FileVersion: 1 
next:20'0                    X error: no match found
          13:  OS/ABI: OpenBSD (0xC) 
next:20'0     ~~~~~~~~~~~~~~~~~~~~~~~
next:20'1      ?                      possible intended match
          14:  ABIVersion: 0 
next:20'0     ~~~~~~~~~~~~~~~
          15:  Unused: (00 00 00 00 00 00 00) 
next:20'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          16:  } 
next:20'0     ~~~
          17:  Type: Executable (0x2) 
next:20'0     ~~~~~~~~~~~~~~~~~~~~~~~~
...

```

https://github.com/llvm/llvm-project/pull/98158