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
 
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<tr><td><span title='2019-12-29 18:24:55 +0100'>2019-12-29</span></td><td><a href='/guix/commit/gnu/installer/final.scm?id=a4ee6ac444ca71a93101b48bc1e6a5710026266c'>installer: Pass '--fallback' to 'guix system init'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-04-28 22:57:07 +0200'>2019-04-28</span></td><td><a href='/guix/commit/gnu/installer/final.scm?id=0e8e963d73e61c7666f9ec4efa98c1a277c72af9'>installer: User accounts can now have a "real name."</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-04-26 14:19:36 +0200'>2019-04-26</span></td><td><a href='/guix/commit/gnu/installer/final.scm?id=9529f7850e3ae91cce70620e684fc6d0dc25e815'>installer: Take 'guix system init' exit code into account.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-04-25 00:45:35 +0200'>2019-04-25</span></td><td><a href='/guix/commit/gnu/installer/final.scm?id=91a7c4998fe4f5a2a63f2ddb4bfeeef81c68b6d7'>installer: Ask for the root account password.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-04-25 00:45:34 +0200'>2019-04-25</span></td><td><a href='/guix/commit/gnu/installer/final.scm?id=898677ed17672130d20434cd88575d620c97c553'>installer: Ask for user password and initialize /etc/shadow.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-04-23 00:34:47 +0200'>2019-04-23</span></td><td><a href='/guix/commit/gnu/installer/final.scm?id=7611074f677f1c3cfe5da426f387eeda1b6ad825'>installer: Run 'guix system init' with the right locale.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>