package/pkg-golang: allow packages to provide extldflags

Currently, only the -static extldflags may be set in the
golang-package infra. However, in some cases, it might be necessary to
pass other flags, either because they are needed on a specific
architecture, or a specific C library; packages may also have a need
to pass arbitrary linker flags when they use CGO linking.

For example, on AArch64, go forcibly uses ld.gold, but it is not
available in all toolchains, and ld.bfd works nowadays (following
patch); another case is musl, where PIE is not supported with go.

Introduce FOO_EXTLDFLAGS, which we use to set those flags, and that
packages can set to pass such flags.

Migrate the current setting of -static to use that new variable.

Signed-off-by: Yann E. MORIN <yann.morin@orange.com>
Tested-by: Nathaniel Roach <nroach44@nroach44.id.au>
Signed-off-by: Fiona Klute (WIWA) <fiona.klute@gmx.de>
Reviewed-by: Yann E. MORIN <yann.morin@orange.com>
Reviewed-by: Christian Stewart <christian@aperture.us>
Signed-off-by: Romain Naour <romain.naour@smile.fr>

docs/manual/adding-packages-golang.adoc

@@ -84,8 +84,10 @@ therefore only use a few of them, or none.
   infrastructure will automatically generate a minimal +go.mod+ file
   in the package source tree if it doesn't exist.
 
-* +FOO_LDFLAGS+ and +FOO_TAGS+ can be used to pass respectively the
-  +LDFLAGS+ or the +TAGS+ to the +go+ build command.
+* +FOO_LDFLAGS+, +FOO_EXTLDFLAGS+, and +FOO_TAGS+ can be used to pass
+  respectively the go +LDFLAGS+ (via the `-ldflags` command line flag),
+  the external linker flags +EXTLDFLAGS+ (via the `-extldflags`
+  command line flag), or the +TAGS+ to the +go+ build command.
 
 * +FOO_BUILD_TARGETS+ can be used to pass the list of targets that
   should be built. If +FOO_BUILD_TARGETS+ is not specified, it