Fixes warnings:

'uninitialized variable'
'coversion from string literal to char *'
Issue eclipse-openj9#897

Signed-off-by: Matthew Hall <matthew.hall3@outlook.com>

runtime/compiler/z/codegen/J9CodeGenerator.cpp

@@ -240,7 +240,7 @@ J9::Z::CodeGenerator::callUsesHelperImplementation(TR::Symbol *sym)
 TR::Linkage *
 J9::Z::CodeGenerator::createLinkage(TR_LinkageConventions lc)
    {
-   TR::Linkage * linkage;
+   TR::Linkage * linkage = nullptr;
    switch (lc)
       {
       case TR_CHelper:
@@ -3259,9 +3259,9 @@ J9::Z::CodeGenerator::checkFieldAlignmentForAtomicLong()
    if (!classBlock)
       return false;
 
-   char* fieldName = "value";
+   const char* fieldName = "value";
    int32_t fieldNameLen = 5;
-   char * fieldSig = "J";
+   const char * fieldSig = "J";
    int32_t fieldSigLen = 1;
    int32_t intOrBoolOffset = self()->fe()->getObjectHeaderSizeInBytes() + self()->fej9()->getInstanceFieldOffset(classBlock, fieldName, fieldNameLen, fieldSig, fieldSigLen);
    return (intOrBoolOffset & 0x3) == 0;