d: Update DIP links in gdc documentation to point at upstream repository

The wiki links probably worked at some point in the distant past, but
now the official location of tracking all D Improvement Proposals is on
the upstream dlang/DIPs GitHub repository.

	PR d/106638

gcc/d/ChangeLog:

	* gdc.texi: Update DIP links to point at upstream dlang/DIPs
	repository.

(cherry picked from commit e56b695)

Author: ibuclaw

gcc/d/gdc.texi

@@ -326,14 +326,17 @@ values are supported:
 @item all
 Turns on all upcoming D language features.
 @item dip1000
-Implements @uref{https://wiki.dlang.org/DIP1000} (Scoped pointers).
+Implements @uref{https://github.com/dlang/DIPs/blob/master/DIPs/other/DIP1000.md}
+(Scoped pointers).
 @item dip1008
-Implements @uref{https://wiki.dlang.org/DIP1008} (Allow exceptions in
-@code{@@nogc} code).
+Implements @uref{https://github.com/dlang/DIPs/blob/master/DIPs/other/DIP1008.md}
+(Allow exceptions in @code{@@nogc} code).
 @item dip1021
-Implements @uref{https://wiki.dlang.org/DIP1021} (Mutable function arguments).
+Implements @uref{https://github.com/dlang/DIPs/blob/master/DIPs/accepted/DIP1021.md}
+(Mutable function arguments).
 @item dip25
-Implements @uref{https://wiki.dlang.org/DIP25} (Sealed references).
+Implements @uref{https://github.com/dlang/DIPs/blob/master/DIPs/archive/DIP25.md}
+(Sealed references).
 @item dtorfields
 Turns on generation for destructing fields of partially constructed objects.
 @item fieldwise
@@ -383,7 +386,8 @@ are supported:
 @item all
 Turns off all revertable D language features.
 @item dip25
-Reverts @uref{https://wiki.dlang.org/DIP25} (Sealed references).
+Reverts @uref{https://github.com/dlang/DIPs/blob/master/DIPs/archive/DIP25.md}
+(Sealed references).
 @item dtorfields
 Turns off generation for destructing fields of partially constructed objects.
 @item markdown