buildman: Update the default settings file

The settings file omits a few lines which are useful for getting every
board building. Add these and update the documentation tool.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/buildman.rst b/tools/buildman/buildman.rst
index afdd65d..4851a64 100644
--- a/tools/buildman/buildman.rst
+++ b/tools/buildman/buildman.rst
@@ -199,6 +199,8 @@
    arc = /opt/arc/arc_gnu_2021.03_prebuilt_elf32_le_linux_install/bin/arc-elf32-
 
    [toolchain-alias]
+   riscv = riscv32
+   sh = sh4
    x86: i386
    blackfin: bfin
    openrisc: or1k