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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<div class='path'>path: <a href='/guix/log/'>root</a>/<a href='/guix/log/gnu'>gnu</a>/<a href='/guix/log/gnu/packages'>packages</a>/<a href='/guix/log/gnu/packages/stb.scm'>stb.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/stb.scm?showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
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<tr><td><span title='2018-11-01 08:20:13 +0100'>2018-11-01</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=ee3dd7a83af94ce19ce4909b15cecc57a5c99cde'>gnu: sdl2: Update to 2.0.9.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sdl.scm (sdl2): Update to 2.0.9.
</span></span></td><td>Rutger Helling</td></tr>
<tr><td><span title='2018-10-27 20:08:50 +0200'>2018-10-27</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=763d44920dc1ae8a1ea2ff5aab58fdf818182880'>gnu: guile-sdl2: Update to 0.3.1.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sdl.scm (guile-sdl2): Update to 0.3.1.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-10-01 12:12:23 +0200'>2018-10-01</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=21be24b74f1a0cac185637620d59a3764c539875'>gnu: Add ghc-sdl2-image.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sdl.scm (ghc-sdl2-image): New variable.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-10-01 12:12:22 +0200'>2018-10-01</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=d975534e856dc6c8426d9f28af3b5d30ead31ac3'>gnu: Add ghc-sdl2-mixer.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sdl.scm (ghc-sdl2-mixer): New variable.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-10-01 12:12:22 +0200'>2018-10-01</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=62dd7171d3fa3df964f34ebb1627704ea447cc21'>gnu: Add ghc-sdl2.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sdl.scm (ghc-sdl2): New variable.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-07-27 14:19:33 +0200'>2018-07-27</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=22305f859b5ce21b46fdca04ca20033ced9ce0e2'>gnu: guile-sdl2: Update to 0.3.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sdl.scm (guile-sdl2): Update to 0.3.0.
[arguments]: Remove "patch-makefile" phase.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-07-18 00:36:07 +0200'>2018-07-18</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=fa3b93d6a7530a0f84a20ad8b96e0efe601da2d1'>gnu: libmikmod: Update to 3.3.11.1.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sdl.scm (libmikmod): Update to 3.3.11.1.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2018-06-21 17:33:13 +0200'>2018-06-21</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=e5bea952ead0b6882bfd2ba2dfbff9f9ed290c8e'>gnu: sdl2-image: Update to 2.0.3.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sdl.scm (sdl2-image): Update to 2.0.3.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2018-06-20 11:28:09 +0200'>2018-06-20</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=e5b77ccfe861545a61f6c6355f5c9fb712bc4894'>gnu: guile-sdl2: Install compiled files to the expected place.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sdl.scm (guile-sdl2)[arguments]: Add build phase
"patch-makefile".
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2018-04-30 04:03:54 -0400'>2018-04-30</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=3d5ad159b336a9903b31d0be7ae052dbc8d5bfcc'>Merge branch 'master' into core-updates</a></td><td>Mark H Weaver</td></tr>