uname.sh: fix cross uname -m when XBPS_ARCH is reseted

From commit cf6e6f14d3, (wrappers/uname: report correct machine type
on ethereal, 2020-02-10).

Some template use `env -i` to reset environment variable for building
host's binary file, thus the wrapped `uname -m` reports empty strings.

Fix it by falling back to the value reported by `/usr/bin/uname -m`.
We don't expect people cross-compile from i686-ethereal-chroot on top
of x86_64 machine, it was expected to use for travis to native compile
i686 only.

common/wrappers/uname.sh

@@ -1,9 +1,13 @@
 #!/bin/sh
 
 uname=$(/usr/bin/uname $@)
+uname_m=$(/usr/bin/uname -m)
+arch=${XBPS_ARCH%-musl}
+# if XBPS_ARCH was reseted by `env -i` use original `/usr/bin/uname -m`
+: ${arch:=$uname_m}
 rv=$?
 echo "$uname" |
 	sed "s/\(^\| \)$(/usr/bin/uname -n)\($\| \)/\1void\2/" |
-	sed "s/$(/usr/bin/uname -m)/${XBPS_ARCH%-musl}/"
+	sed "s/$uname_m/$arch/"
 
 exit $rv