Fix assigning to _gp in linker scripts.

The previous logic was to try to detect if a linker script defined _gp
by checking !ElfSym::MipsGp->Value. That doesn't work in all cases as
the assigned value can be 0.

We now just always defined it Writer.cpp and always overwrite it
afterwards if needed.

llvm-svn: 313788
diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 6dd3d9f..b4342a9 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -964,7 +964,7 @@
 
   // Setup MIPS _gp_disp/__gnu_local_gp symbols which should
   // be equal to the _gp symbol's value.
-  if (ElfSym::MipsGp && !ElfSym::MipsGp->Value) {
+  if (ElfSym::MipsGp) {
     // Find GP-relative section with the lowest address
     // and use this address to calculate default _gp value.
     for (OutputSection *OS : OutputSections) {