Fix #69 (No way to set globally a custom architecture)

The "architecture" configuration keyword is now available to override
the native machine architecture returned by (uname(2)).

The XBPS_ARCH environment variable still has preference.

bin/xbps-uhelper/main.c

@@ -45,7 +45,7 @@ usage(void)
 	"  Available actions:\n"
 	"    binpkgarch, binpkgver, cmpver, digest, fetch, getpkgdepname,\n"
 	"    getpkgname, getpkgrevision, getpkgversion, pkgmatch, version,\n"
-	"    real-version.\n"
+	"    real-version, arch.\n"
 	"\n"
 	"  Action arguments:\n"
 	"    binpkgarch\t<binpkg>\n"
@@ -138,6 +138,7 @@ main(int argc, char **argv)
 
 	if ((strcmp(argv[0], "version") == 0) ||
 	    (strcmp(argv[0], "real-version") == 0) ||
+	    (strcmp(argv[0], "arch") == 0) ||
 	    (strcmp(argv[0], "fetch") == 0)) {
 		/*
 		* Initialize libxbps.
@@ -269,6 +270,12 @@ main(int argc, char **argv)
 			usage();
 
 		exit(xbps_cmpver(argv[1], argv[2]));
+	} else if (strcmp(argv[0], "arch") == 0) {
+		/* returns the xbps native arch */
+		if (argc != 1)
+			usage();
+
+		printf("%s\n", xh.native_arch);
 	} else if (strcmp(argv[0], "digest") == 0) {
 		/* Prints SHA256 hashes for specified files */
 		if (argc < 2)