[gtk-osx] Revert architecture detection to uname -a.

[John Ralls <jralls src gnome org>]

commit 3a97b87abd3b85f9df18031c9900388f4685c061
Author: John Ralls <jralls ceridwen us>
Date:   Tue Sep 22 13:02:16 2020 -0700

    Revert architecture detection to uname -a.
    
    The mod was a workaround for building in a Rosetta'd terminal program.

 jhbuildrc-gtk-osx | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)
---
diff --git a/jhbuildrc-gtk-osx b/jhbuildrc-gtk-osx
index 5572ab7..ef85b49 100644
--- a/jhbuildrc-gtk-osx
+++ b/jhbuildrc-gtk-osx
@@ -127,12 +127,8 @@ def osx_ver():
     vernum = exp.match(vstring)
     _osx_version = float(vernum.group(1)) - 4.0
     _target = '10.%d' % int(_osx_version)
-    
-    is_arm = _popen("uname -v").endswith("RELEASE_ARM64_T8020")
-    if is_arm:
-        _default_arch = "arm64"
-    else:
-        _default_arch = "x86_64"
+
+    _default_arch = _popen("uname -m")
 
 # Determine the XCode Version:
 def xcode_ver():