[gardening] Fix gcc and msvc builds

Fix the following compile-time errors:

C:\b\s\w\ir\cache\builder\sdk\runtime\vm\elf.cc(601) : error C2220: the following warning is treated as an error
C:\b\s\w\ir\cache\builder\sdk\runtime\vm\elf.cc(601) : warning C4715: 'dart::ElfSymbolTable::ElfSymbolType': not all control paths return a value

../../runtime/vm/elf.cc: In static member function ‘static intptr_t dart::ElfSymbolTable::ElfSymbolType(dart::SharedObjectWriter::SymbolData::Type)’:
../../runtime/vm/elf.cc:601:3: error: control reaches end of non-void function [-Werror=return-type]
  601 |   }
      |   ^

TEST=ci

Change-Id: Ib06339405b406ea4a6f6d395a04a16573d92c704
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/430321
Auto-Submit: Alexander Markov <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: alexmarkov

runtime/vm/elf.cc

@@ -598,6 +598,7 @@ class ElfSymbolTable : public ElfSection {
       case SharedObjectWriter::SymbolData::Type::Object:
         return elf::STT_OBJECT;
     }
+    UNREACHABLE();
   }
 
   Zone* const zone_;