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='2020-02-02 10:42:16 +0100'>2020-02-02</span></td><td><a href='/guix/commit/gnu/services/pm.scm?id=8a890f3c40f698c976dc2a759ccc9c11641bd4f6'>gnu: tlp: Update to 1.3.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/linux.scm (tlp): Update to 1.3.0.
[source]: Hard-code name in origin URI.
[native-inputs]: Add "shellcheck".
[arguments]: Activate some tests.  Update environment variables.  Ignore
tlp-rdw man pages.
* gnu/services/pm.scm (tlp-activation): Update configuration file name.
</span></span></td><td>Nicolas Goaziou</td></tr>
<tr><td><span title='2019-08-26 13:40:45 +0200'>2019-08-26</span></td><td><a href='/guix/commit/gnu/services/pm.scm?id=94b98ef7a089d7a5720a9af2369dc994eb7ffe9d'>services: tlp, thermald: Add descriptions.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/pm.scm (tlp-service-type)[description]: New field.
(thermald-service-type)[description]: New field.
</span></span></td><td>Ludovic Courtès</td></tr>