[ELF] Provide the GNU hash function in libObject

GNU uses a different hashing function compared to the sys-V standard
function already provided in libObject. This is already used internally
in LLD for generating synthetic sections. This patch simply extracts
this definition and makes it availible to other users of `libObject`.
This is done in preparation for supporting symbol name lookups via the
GNU hash table.

Reviewed By: MaskRay, jhenderson

Differential Revision: https://reviews.llvm.org/D132696

lld/ELF/SyntheticSections.cpp

@@ -2397,13 +2397,6 @@ void GnuHashTableSection::writeTo(uint8_t *buf) {
   }
 }
 
-static uint32_t hashGnu(StringRef name) {
-  uint32_t h = 5381;
-  for (uint8_t c : name)
-    h = (h << 5) + h + c;
-  return h;
-}
-
 // Add symbols to this symbol hash table. Note that this function
 // destructively sort a given vector -- which is needed because
 // GNU-style hash table places some sorting requirements.

llvm/include/llvm/Object/ELF.h

@@ -1223,6 +1223,16 @@ inline unsigned hashSysV(StringRef SymbolName) {
   return h;
 }
 
+/// This function returns the hash value for a symbol in the .dynsym section
+/// for the GNU hash table. The implementation is defined in the GNU hash ABI.
+/// REF : https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elf.c#l222
+inline uint32_t hashGnu(StringRef Name) {
+  uint32_t H = 5381;
+  for (uint8_t C : Name)
+    H = (H << 5) + H + C;
+  return H;
+}
+
 } // end namespace object
 } // end namespace llvm