Use `git rev-parse` to find .git dir

Otherwise, this code breaks if riscv-gnu-toolchain is itself a
submodule of another project. In that case the git config for this
tree will be $(srcdir)/../.git/modules/riscv-gnu-toolchain/config.

Makefile.in

@@ -245,8 +245,8 @@ endif
 
 $(srcdir)/%/.git:
 	cd $(srcdir) && \
-	flock $(srcdir)/.git/config git submodule init $(dir $@) && \
-	flock $(srcdir)/.git/config git submodule update $(dir $@)
+	flock `git rev-parse --git-dir`/config git submodule init $(dir $@) && \
+	flock `git rev-parse --git-dir`/config git submodule update $(dir $@)
 
 #
 # GLIBC