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='2020-12-24 15:56:57 -0500'>2020-12-24</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=88795880d79993e18e0ea5b2c90a3dcf7488d16b'>gnu: plantuml: Update to 1.2020.24.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/uml.scm (plantuml): Update to 1.2020.24.

Signed-off-by: Leo Famulari &lt;leo@famulari.name&gt;
</span></span></td><td>Michael Rohleder</td></tr>
<tr><td><span title='2020-10-14 16:31:45 +0200'>2020-10-14</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=2d0bea9c6d98a0dec99218935239dca8b676b86f'>gnu: plantuml: Update to 1.2020.19.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/uml.scm (plantuml): Update to 1.2020.19.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-09-24 09:16:50 +0200'>2020-09-24</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=89ba5f9479a001c89872e42bd1ffdc82bf02b5f7'>gnu: plantuml: Update to 1.2020.17.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/uml.scm (plantuml): Update to 1.2020.17.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-09-18 11:18:09 +0200'>2020-09-18</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=6182897c98e11676b9a370da297a688c311f4cab'>gnu: plantuml: Update to 1.2020.16.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/uml.scm (plantuml): Update to 1.2020.16.

Signed-off-by: Mathieu Othacehe &lt;othacehe@gnu.org&gt;
</span></span></td><td>Michael Rohleder</td></tr>
<tr><td><span title='2020-08-01 09:13:40 +0200'>2020-08-01</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=033b43b2684334655e9c778d8a63f4f61561e759'>gnu: plantuml: Update to 1.2020.15.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/uml.scm (plantuml): Update to 1.2020.15.

Signed-off-by: Mathieu Othacehe &lt;othacehe@gnu.org&gt;
</span></span></td><td>Michael Rohleder</td></tr>