[Backtracing] Fix symbol lookup on Linux.

Also removed a spurious `print()` that got left in by accident,
and disabled the `CodableBacktrace.swift` test on Linux since we
need Foundation and we don't have it there.

rdar://124913332

Author: al45tair

stdlib/public/RuntimeModule/Backtrace.swift

@@ -415,8 +415,6 @@ extension Backtrace {
                                memoryReader: memoryReader)
 
         if let limit = limit {
-          print("limit = \(limit), offset = \(offset), top = \(top)")
-
           let limited = LimitSequence(unwinder,
                                       limit: limit,
                                       offset: offset,

stdlib/public/RuntimeModule/Elf.swift

@@ -1156,13 +1156,13 @@ struct ElfSymbolTable<SomeElfTraits: ElfTraits>: ElfSymbolTableProtocol {
         nextValue = _symbols[mid + 1].value
       }
 
-      if symbol.value <= address && nextValue >= address {
+      if symbol.value <= address && nextValue > address {
         var ndx = mid
         while ndx > 0 && _symbols[ndx - 1].value == address {
           ndx -= 1
         }
         return _symbols[ndx]
-      } else if symbol.value < address {
+      } else if symbol.value <= address {
         min = mid + 1
       } else if symbol.value > address {
         max = mid

test/Backtracing/CodableBacktrace.swift

@@ -5,7 +5,7 @@
 
 // REQUIRES: executable_test
 // REQUIRES: backtracing
-// REQUIRES: OS=macosx || OS=linux-gnu
+// REQUIRES: OS=macosx
 
 // UNSUPPORTED: use_os_stdlib
 // UNSUPPORTED: back_deployment_runtime