Merge bitcoin/bitcoin#27668: guix: document when certain patches can be dropped

a09269a guix: document when certain guix patches can be dropped (fanquake)

Pull request description:

  Additional notes for when patches can be dropped.

ACKs for top commit:
  hebasto:
    ACK a09269a, I have reviewed the changes and they look OK.
  jarolrod:
    ACK a09269a

Tree-SHA512: c1876b9a4e3cf73645d25c9077cef19a9b6b7fe2eda5dc9d82fd3ca3f9105453406c1b197e6635035b6ce19c9f255c070bebed5563f68913033d04627202155a

Author: fanquake

contrib/guix/patches/gcc-broken-longjmp.patch

@@ -29,6 +29,8 @@ Date:   Wed May 5 22:48:51 2021 +0200
     gcc/testsuite/
             * gcc.c-torture/execute/20210505-1.c: New test.
 
+    This patch can be dropped when we are building with GCC 10.4.0 or later.
+
 diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
 index 2f838840e96..06ad1b2274e 100644
 --- a/gcc/config/i386/i386.c

contrib/guix/patches/glibc-2.27-fcommon.patch

@@ -5,7 +5,7 @@ Date:   Fri May 6 11:03:04 2022 +0100
     build: use -fcommon to retain legacy behaviour with GCC 10
 
     GCC 10 started using -fno-common by default, which causes issues with
-    the powerpc builds using gibc 2.24. A patch was commited to glibc to fix
+    the powerpc builds using gibc 2.27. A patch was commited to glibc to fix
     the issue, 18363b4f010da9ba459b13310b113ac0647c2fcc but is non-trvial
     to backport, and was broken in at least one way, see the followup in
     commit 7650321ce037302bfc2f026aa19e0213b8d02fe6.
@@ -17,6 +17,8 @@ Date:   Fri May 6 11:03:04 2022 +0100
     https://sourceware.org/git/?p=glibc.git;a=commit;h=18363b4f010da9ba459b13310b113ac0647c2fcc
     https://sourceware.org/git/?p=glibc.git;a=commit;h=7650321ce037302bfc2f026aa19e0213b8d02fe6
 
+    This patch can be dropped when we are building with glibc 2.31+.
+
 diff --git a/Makeconfig b/Makeconfig
 index 86a71e5802..aa2166be60 100644
 --- a/Makeconfig

contrib/guix/patches/glibc-2.27-guix-prefix.patch

@@ -5,7 +5,7 @@ In order to be reproducible regardless of the architecture used to build
 the package, map all guix store prefixes to something fixed, e.g. /usr.
 
 We might be able to drop this in favour of using --with-nonshared-cflags
-when we being using newer versions of glibc.
+when we begin using newer versions of glibc.
 
 --- a/Makeconfig
 +++ b/Makeconfig

contrib/guix/patches/glibc-2.27-riscv64-Use-__has_include-to-include-asm-syscalls.h.patch

@@ -4,6 +4,8 @@ See also: http://lists.busybox.net/pipermail/buildroot/2020-July/590376.html.
 
 https://sourceware.org/git/?p=glibc.git;a=commit;h=0b9c84906f653978fb8768c7ebd0ee14a47e662e
 
+This patch can be dropped when we are building with glibc 2.28+.
+
 From 562c52cc81a4e456a62e6455feb32732049e9070 Mon Sep 17 00:00:00 2001
 From: "H.J. Lu" <[email protected]>
 Date: Mon, 31 Dec 2018 09:26:42 -0800

contrib/guix/patches/vmov-alignment.patch

@@ -1,6 +1,7 @@
 Description: Use unaligned VMOV instructions
 Author: Stephen Kitt <[email protected]>
 Bug-Debian: https://bugs.debian.org/939559
+See also: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54412
 
 Based on a patch originally by Claude Heiland-Allen <[email protected]>