Use 'uname -m', which in practice returns the processor architecture.

--- psm-3.3/buildflags.mak~	2014-08-20 21:00:18.000000000 +0100
+++ psm-3.3/buildflags.mak	2017-10-22 12:02:12.855291447 +0100
@@ -38,7 +38,7 @@
 endif
 
 export os ?= $(shell uname -s | tr '[A-Z]' '[a-z]')
-export arch := $(shell uname -p | sed -e 's,\(i[456]86\|athlon$$\),i386,')
+export arch := $(shell uname -m | sed -e 's,\(i[456]86\|athlon$$\),i386,')
 
 CC ?= gcc
 
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<tr><td><span title='2022-09-04 23:10:10 +0200'>2022-09-04</span></td><td><a href='/guix/commit/tests/guix-describe.sh?id=9e59dbcd6cc37ee65236edf23a9b32338536e036'>guix describe: Adjust test to latest changes.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a followup to 64a070717c3de32332201df5d6d2d52a7f99dce9.

* tests/guix-describe.sh: Add trailing slash when checking URL.
</span></span></td><td>Ludovic Courtès</td></tr>