limit building GNU assembler (as) to x86

GNU as 2.17.50 is currently required by amd64 and i386 for at least one
file that cannot be assembled by Clang's integrated assembler (IAS).
Other supported CPU architectures either use Clang IAS for all assembly
files, or rely on external toolchain.

Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D23180

gnu/usr.bin/binutils/Makefile

@@ -8,9 +8,15 @@ SUBDIR=	libiberty \
 
 SUBDIR.${MK_BINUTILS}+=	doc
 SUBDIR.${MK_BINUTILS}+=	libbinutils
-SUBDIR.${MK_BINUTILS}+=	as
 SUBDIR.${MK_BINUTILS}+=	objdump
 
+# GNU as is used on x86 only, for a few files that cannot be assembled by
+# Clang IAS. Other archs either use Clang IAS for every assembly file, or
+# use external toolchain.
+.if ${TARGET} == "amd64" || ${TARGET} == "i386"
+SUBDIR.${MK_BINUTILS}+=	as
+.endif
+
 # All archs except powerpc either use lld or require external toolchain.
 # powerpc still needs binutils ld to link 32-bit binaries.
 .if ${TARGET} == "powerpc"

tools/build/options/WITH_BINUTILS

@@ -1,8 +1,8 @@
 .\" $FreeBSD$
-Set to build and install GNU
-.Xr as 1 ,
+Build and install GNU
+.Xr as 1
+on i386 and amd64,
 .Xr objdump 1 ,
-and, on powerpc,
+and
 .Xr ld.bfd 1
-as part
-of the normal system build.
+on powerpc as part of the normal system build.

tools/build/options/WITH_BINUTILS_BOOTSTRAP

@@ -1,3 +1,3 @@
 .\" $FreeBSD$
-Set build binutils (as, objdump, and on powerpc ld)
+Build binutils (as on i386 and amd64, objdump, and ld on powerpc)
 as part of the bootstrap process.