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='2017-01-23 17:23:03 -0500'>2017-01-23</span></td><td><a href='/guix/commit/gnu/packages/orpheus.scm?id=147c6b391b3264fa6606841452e2c8049a01d983'>gnu: orpheus: Don't replace 'sh' reference with 'bash'.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2016-09-23 17:38:25 +0300'>2016-09-23</span></td><td><a href='/guix/commit/gnu/packages/orpheus.scm?id=047cd01d11539b4afc547e430ae7f3b9af976bea'>gnu: orpheus: Use 'modify-phases'.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2016-09-23 17:38:24 +0300'>2016-09-23</span></td><td><a href='/guix/commit/gnu/packages/orpheus.scm?id=5bd6270211b8b4d9335960eb5741c34308187ccd'>gnu: orpheus: Fix build on mips64el.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>