Default GCC debug format to DWARF 4

Even the latest GDB release does not correctly support DWARF 5 from GCC,
most notably preprocessor macros. This is a bug in either GCC or GDB and
is present across every platform tested. DWARF 4 works fine, so stick to
it for now.

Dockerfile

@@ -88,7 +88,9 @@ WORKDIR /bootstrap
 RUN ln -s $ARCH mingw
 
 WORKDIR /x-gcc
-RUN /gcc-$GCC_VERSION/configure \
+COPY src/gcc-*.patch $PREFIX/src/
+RUN cat $PREFIX/src/gcc-*.patch | patch -d/gcc-$GCC_VERSION -p1 \
+ && /gcc-$GCC_VERSION/configure \
         --prefix=/bootstrap \
         --with-sysroot=/bootstrap \
         --target=$ARCH \

src/gcc-000-dwarf4.patch

@@ -0,0 +1,7 @@
+--- a/gcc/common.opt
++++ b/gcc/common.opt
+@@ -3290,3 +3290,3 @@
+ gdwarf-
+-Common Driver Joined UInteger Var(dwarf_version) Init(5) Negative(gstabs)
++Common Driver Joined UInteger Var(dwarf_version) Init(4) Negative(gstabs)
+ Generate debug information in DWARF v2 (or later) format.