Introduce -nostdlib++ option

Using g++ to link without libstdc++, as in g++.dg/abi/pure-virtual1.C,
is error prone, because there's no way to tell g++ to drop libstdc++
without also dropping libc and any other libraries that the target
implicitly links in.

This has often led to the need for manual adjustments to this
testcase.

I figured adding support for -nostdlib++, even though redundant, makes
some sense.  One could presumably use gcc rather than g++ for linking,
for the same effect, but sometimes changing the link command is harder
than adding an option, as in our testsuite.

Since clang already had an option with this effect, we've adopted the
same spelling.


for  gcc/ChangeLog

	* common.opt (nostdlib++): New.
	* doc/invoke.texi (-nostdlib++): Document it.

for  gcc/cp/ChangeLog

	* g++spec.cc (lang_specific_driver): Implement -nostdlib++.

for  gcc/testsuite/ChangeLog

	* g++.dg/abi/pure-virtual1.C: Use -nostdlib++.

gcc/common.opt

@@ -3467,6 +3467,9 @@ Driver
 nostdlib
 Driver
 
+nostdlib++
+Driver
+
 o
 Common Driver Joined Separate Var(asm_file_name) MissingArgError(missing filename after %qs)
 -o <file>	Place output into <file>.

gcc/cp/g++spec.cc

@@ -159,6 +159,7 @@ lang_specific_driver (struct cl_decoded_option **in_decoded_options,
       switch (decoded_options[i].opt_index)
 	{
 	case OPT_nostdlib:
+	case OPT_nostdlib__:
 	case OPT_nodefaultlibs:
 	  library = -1;
 	  break;

gcc/doc/invoke.texi

@@ -653,7 +653,7 @@ Objective-C and Objective-C++ Dialects}.
 @item Linker Options
 @xref{Link Options,,Options for Linking}.
 @gccoptlist{@var{object-file-name}  -fuse-ld=@var{linker}  -l@var{library} @gol
--nostartfiles  -nodefaultlibs  -nolibc  -nostdlib @gol
+-nostartfiles  -nodefaultlibs  -nolibc  -nostdlib  -nostdlib++ @gol
 -e @var{entry}  --entry=@var{entry} @gol
 -pie  -pthread  -r  -rdynamic @gol
 -s  -static  -static-pie  -static-libgcc  -static-libstdc++ @gol
@@ -16846,6 +16846,10 @@ library subroutines.
 constructors are called; @pxref{Collect2,,@code{collect2}, gccint,
 GNU Compiler Collection (GCC) Internals}.)
 
+@item -nostdlib++
+@opindex nostdlib++
+Do not implicitly link with standard C++ libraries.
+
 @item -e @var{entry}
 @itemx --entry=@var{entry}
 @opindex e

gcc/testsuite/g++.dg/abi/pure-virtual1.C

@@ -1,7 +1,7 @@
 // Test that we don't need libsupc++ just for __cxa_pure_virtual.
 // { dg-do link }
 // { dg-require-weak }
-// { dg-additional-options "-fno-rtti -nodefaultlibs -lc" }
+// { dg-additional-options "-fno-rtti -nostdlib++" }
 // { dg-additional-options "-Wl,-undefined,dynamic_lookup" { target *-*-darwin* } }
 // { dg-xfail-if "AIX weak" { powerpc-ibm-aix* } }