gollvm: Use aarch64linux emulation mode variants

When using the default ld linker on linux arm64, library libpthread.so can't be found.
Because currently we set emulation mode as aarch64elf, and its default search directories
only contain "/usr/aarch64-unknown-linux-gnu/lib", but libpthread.so is not there. We
can fix this issue by adding the correct path of this library to search list, but I'm
not sure if the path is fixed. As now we only support arm64 on linux, we simply replace
aarch64elf emulation mode with aarch64linux.

Change-Id: Ie779410d2e38f8d928a47cf31c1d68900eb19997
Reviewed-on: https://go-review.googlesource.com/c/gollvm/+/232757
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/driver/GnuTools.cpp b/driver/GnuTools.cpp
index 3748e10..8719a4a 100644
--- a/driver/GnuTools.cpp
+++ b/driver/GnuTools.cpp
@@ -215,7 +215,8 @@
     case llvm::Triple::aarch64:
-      cmdArgs.push_back("aarch64elf");
+      // Currently only support linux/arm64
+      cmdArgs.push_back("aarch64linux");
       // unhandled architecture