ref-manual/variables.rst: document HOST_*_ARCH variables

These variables control the flags for the assembler, compiler and
linker, but depend on the context.

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
(From yocto-docs rev: cfc7bce0b7368a6ecfaef7c7df6222f1a6076e9b)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit f8eb33569a5e8cadc036855e2d95eee77e627cb4)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -3585,6 +3585,20 @@ system and gives an overview of their function and contents.
       - mips
       - mipsel
 
+   :term:`HOST_AS_ARCH`
+      Specifies architecture-specific assembler flags.
+
+      Default initialization for :term:`HOST_AS_ARCH` varies depending on what
+      is being built:
+
+      -  :term:`TARGET_AS_ARCH` when building for the
+         target
+
+      -  :term:`BUILD_AS_ARCH` when building for the build host (i.e.
+         ``-native``)
+
+      -  :term:`SDK_AS_ARCH` when building for an SDK (i.e. ``nativesdk-``)
+
    :term:`HOST_CC_ARCH`
       Specifies architecture-specific compiler flags that are passed to the
       C compiler.
@@ -3601,6 +3615,19 @@ system and gives an overview of their function and contents.
       -  ``BUILDSDK_CC_ARCH`` when building for an SDK (i.e.
          ``nativesdk-``)
 
+   :term:`HOST_LD_ARCH`
+      Specifies architecture-specific linker flags.
+
+      Default initialization for :term:`HOST_LD_ARCH` varies depending on what
+      is being built:
+
+      -  :term:`TARGET_LD_ARCH` when building for the target
+
+      -  :term:`BUILD_LD_ARCH` when building for the build host (i.e.
+         ``-native``)
+
+      -  :term:`SDK_LD_ARCH` when building for an SDK (i.e. ``nativesdk-``)
+
    :term:`HOST_OS`
       Specifies the name of the target operating system, which is normally
       the same as the :term:`TARGET_OS`. The variable can