Support cross compiling kernels with GCC6

• aportgen: modify code to allow generating gcc6-armhf and other gcc6 cross compiler packages
• package: when 'gcc6' is in the depends of a package, and the cross compiling mode is "native" (as we do it with kernels), install the gcc6 cross compiler instead of the usual one (gcc8)

(reviews would be nice, but this is so urgent that I'll continue ASAP anyway even if nobody reviews it. I know that this will probably not pass CI right now.)

Related: pmaports#103 (closed)

pmb/aportgen/gcc.py

@@ -23,8 +23,17 @@ import pmb.helpers.run
 
 def generate(args, pkgname):
     # Copy original aport
+    prefix = pkgname.split("-")[0]
     arch = pkgname.split("-")[1]
-    upstream = pmb.aportgen.core.get_upstream_aport(args, "main/gcc")
+    if prefix == "gcc":
+        upstream = pmb.aportgen.core.get_upstream_aport(args, "main/gcc")
+        based_on = "main/gcc (from Alpine)"
+    elif prefix == "gcc6":
+        upstream = args.aports + "/main/gcc6"
+        based_on = "main/gcc6 (from postmarketOS)"
+    else:
+        raise ValueError("Invalid prefix '" + prefix + "', expected gcc or"
+                         " gcc6.")
     pmb.helpers.run.user(args, ["cp", "-r", upstream, args.work + "/aportgen"])
 
     # Architectures to build this package for
@@ -39,7 +48,11 @@ def generate(args, pkgname):
         "depends": "isl binutils-" + arch,
         "makedepends_build": "gcc g++ paxmark bison flex texinfo gawk zip gmp-dev mpfr-dev mpc1-dev zlib-dev",
         "makedepends_host": "linux-headers gmp-dev mpfr-dev mpc1-dev isl-dev zlib-dev musl-dev-" + arch + " binutils-" + arch,
-        "subpackages": "g++-" + arch + ":gpp",
+        "subpackages": "g++-" + arch + ":gpp" if prefix == "gcc" else "",
+
+        # gcc6: options is already there, so we need to replace it and not only
+        # set it below the header like done below.
+        "options": "!strip !tracedeps",
 
         "LIBGOMP": "false",
         "LIBGCC": "false",
@@ -80,6 +93,6 @@ def generate(args, pkgname):
         '*_cross_configure=*': None,
     }
 
-    pmb.aportgen.core.rewrite(args, pkgname, "main/gcc", fields,
+    pmb.aportgen.core.rewrite(args, pkgname, based_on, fields,
                               replace_simple=replace_simple,
                               below_header=below_header)