kbuild: get rid of misleading $(AS) from documents

The assembler files in the kernel are *.S instead of *.s, so they must
be preprocessed. Since 'as' of GNU binutils is not able to preprocess,
we always use $(CC) as an assembler driver.

$(AS) is almost unused in Kbuild. As of v5.2, there is just one place
that directly invokes $(AS).

  $ git grep -e '$(AS)' -e '${AS}' -e '$AS' -e '$(AS:' -e '${AS:' -- :^Documentation
  drivers/net/wan/Makefile:  AS68K = $(AS)

The documentation about *_AFLAGS* sounds like the flags were passed
to $(AS). This is somewhat misleading.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>

Documentation/kbuild/kbuild.rst

@@ -38,12 +38,11 @@ Additional options to the assembler (for built-in and modules).
 
 AFLAGS_MODULE
 -------------
-Additional module specific options to use for $(AS).
+Additional assembler options for modules.
 
 AFLAGS_KERNEL
 -------------
-Additional options for $(AS) when used for assembler
-code for code that is compiled as built-in.
+Additional assembler options for built-in.
 
 KCFLAGS
 -------

Documentation/kbuild/makefiles.rst

@@ -328,7 +328,7 @@ more details, with real examples.
 	variable $(KBUILD_CFLAGS) and uses it for compilation flags for the
 	entire tree.
 
-	asflags-y specifies options for assembling with $(AS).
+	asflags-y specifies assembler options.
 
 	Example::
 
@@ -489,7 +489,7 @@ more details, with real examples.
 	as-instr checks if the assembler reports a specific instruction
 	and then outputs either option1 or option2
 	C escapes are supported in the test instruction
-	Note: as-instr-option uses KBUILD_AFLAGS for $(AS) options
+	Note: as-instr-option uses KBUILD_AFLAGS for assembler options
 
     cc-option
 	cc-option is used to check if $(CC) supports a given option, and if
@@ -905,7 +905,7 @@ When kbuild executes, the following steps are followed (roughly):
 	vmlinux. The usage of $(call if_changed,xxx) will be described later.
 
     KBUILD_AFLAGS
-	$(AS) assembler flags
+	Assembler flags
 
 	Default value - see top level Makefile
 	Append or modify as required per architecture.
@@ -948,16 +948,16 @@ When kbuild executes, the following steps are followed (roughly):
 	to 'y' when selected.
 
     KBUILD_AFLAGS_KERNEL
-	$(AS) options specific for built-in
+	Assembler options specific for built-in
 
 	$(KBUILD_AFLAGS_KERNEL) contains extra C compiler flags used to compile
 	resident kernel code.
 
     KBUILD_AFLAGS_MODULE
-	Options for $(AS) when building modules
+	Assembler options specific for modules
 
 	$(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
-	are used for $(AS).
+	are used for assembler.
 
 	From commandline AFLAGS_MODULE shall be used (see kbuild.txt).