Fall back to the portable pthread coroutine backend on CPUs that have no
QuickThreads assembly in the autotools build (arm, riscv, ppc, ...), instead
of aborting configure with "sorry...architecture not supported".

https://bugs.gentoo.org/926545
--- a/configure.ac
+++ b/configure.ac
@@ -161,7 +161,11 @@
          [TARGET_ARCH="linuxaarch64"
           QT_ARCH="aarch64"
           CPU_ARCH=""],
-       [AC_MSG_ERROR("sorry...architecture not supported")])
+       dnl Other CPUs (arm, riscv, ppc, ...) have no QuickThreads assembly in
+       dnl the autotools build; fall back to the portable pthread coroutine
+       dnl backend instead of aborting.  See https://bugs.gentoo.org/926545
+       [QT_ARCH="pthreads"
+        CPU_ARCH=""])
 
      # check compiler
      AS_VAR_IF([GXX],[yes],dnl