daemon: Allow building for armhf-linux on aarch64-linux.

* nix/libstore/build.cc (canBuildLocally): Allow building armhf-linux
builds on aarch64-linux.
(DerivationGoal::runChild) Throw error if attempting to build for
armhf-linux on an unsupported platform.
* doc/guix.texi (Invoking guix build): Document how to build natively
for armhf-linux on aarch64-linux.  Add note that on some aarch64
machines this is unsupported.

doc/guix.texi

@@ -5855,9 +5855,16 @@ information on cross-compilation.
 
 An example use of this is on Linux-based systems, which can emulate
 different personalities.  For instance, passing
-@code{--system=i686-linux} on an @code{x86_64-linux} system allows you
+@code{--system=i686-linux} on an @code{x86_64-linux} system or
+@code{--system=armhf-linux} on an @code{aarch64-linux} system allows you
 to build packages in a complete 32-bit environment.
 
+@quotation Note
+Building for an @code{armhf-linux} system is unconditionally enabled on
+@code{aarch64-linux} machines, although certain aarch64 chipsets do not
+allow for this functionality, notably the ThunderX.
+@end quotation
+
 Similarly, when transparent emulation with QEMU and @code{binfmt_misc}
 is enabled (@pxref{Virtualization Services,
 @code{qemu-binfmt-service-type}}), you can build for any system for

nix/libstore/build.cc

@@ -1244,6 +1244,7 @@ static bool canBuildLocally(const string & platform)
     return platform == settings.thisSystem
 #if __linux__
         || (platform == "i686-linux" && settings.thisSystem == "x86_64-linux")
+        || (platform == "armhf-linux" && settings.thisSystem == "aarch64-linux")
 #endif
         ;
 }
@@ -2220,6 +2221,13 @@ void DerivationGoal::runChild()
                 throw SysError("cannot set i686-linux personality");
         }
 
+        if (drv.platform == "armhf-linux" &&
+            (settings.thisSystem == "aarch64-linux" ||
+             (!strcmp(utsbuf.sysname, "Linux") && !strcmp(utsbuf.machine, "aarch64")))) {
+            if (personality(PER_LINUX32) == -1)
+                throw SysError("cannot set armhf-linux personality");
+        }
+
         /* Impersonate a Linux 2.6 machine to get some determinism in
            builds that depend on the kernel version. */
         if ((drv.platform == "i686-linux" || drv.platform == "x86_64-linux") && settings.impersonateLinux26) {