The patch fixes build failure of form:
     ../src/alpha/osf.S:298:2: error: #error "osf.S out of sync with ffi.h"
Upstream fixed the bug in a more invasive way
but didn't have releases since 3.2.1.

The patch is taken from Gentoo:
https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-libs/libffi/files/libffi-3.2.1-complex_alpha.patch

--- libffi-3.2.1/src/alpha/osf.S	2015-01-16 10:46:15.000000000 +0100
+++ libffi-3.2.1/src/alpha/osf.S	2015-01-16 10:46:24.000000000 +0100
@@ -279,6 +279,7 @@
 	.gprel32 $load_64	# FFI_TYPE_SINT64
 	.gprel32 $load_none	# FFI_TYPE_STRUCT
 	.gprel32 $load_64	# FFI_TYPE_POINTER
+	.gprel32 $load_none	# FFI_TYPE_COMPLEX
 
 /* Assert that the table above is in sync with ffi.h.  */
 
@@ -294,7 +295,8 @@
 	|| FFI_TYPE_SINT64 != 12	\
 	|| FFI_TYPE_STRUCT != 13	\
 	|| FFI_TYPE_POINTER != 14	\
-	|| FFI_TYPE_LAST != 14
+	|| FFI_TYPE_COMPLEX != 15       \
+	|| FFI_TYPE_LAST != 15
 #error "osf.S out of sync with ffi.h"
 #endif
 
 class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=3211f45f0b741d5dea97de3745804881d7446d34'>refs</a><a class='active' href='/guix/log/gnu/packages/valgrind.scm'>log</a><a href='/guix/tree/gnu/packages/valgrind.scm?id=3211f45f0b741d5dea97de3745804881d7446d34'>tree</a><a href='/guix/commit/gnu/packages/valgrind.scm?id=3211f45f0b741d5dea97de3745804881d7446d34'>commit</a><a href='/guix/diff/gnu/packages/valgrind.scm?id=3211f45f0b741d5dea97de3745804881d7446d34'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/valgrind.scm'>
<input type='hidden' name='id' value='3211f45f0b741d5dea97de3745804881d7446d34'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=3211f45f0b741d5dea97de3745804881d7446d34'>root</a>/<a href='/guix/log/gnu?id=3211f45f0b741d5dea97de3745804881d7446d34'>gnu</a>/<a href='/guix/log/gnu/packages?id=3211f45f0b741d5dea97de3745804881d7446d34'>packages</a>/<a href='/guix/log/gnu/packages/valgrind.scm?id=3211f45f0b741d5dea97de3745804881d7446d34'>valgrind.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/valgrind.scm?id=3211f45f0b741d5dea97de3745804881d7446d34&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-03-06 16:30:48 +0100'>2019-03-06</span></td><td><a href='/guix/commit/gnu/packages/valgrind.scm?id=a9eeda20b4eb782941350a99e0331d33c5ad4d28'>gnu: valgrind: Add download mirror.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>