gcc-source: Fix incorrect task dependencies from ${B}

Some tasks may reference ${B} for gcc-source which in general would not exist.
It has dependencies on HOST_SYS and TARGET_SYS which are not appropriate for a
shared recipe like gcc-source. This causes problems for the archiver and
multiconfigs in particlar.

Set B to something else to avoid these task hash issues.

Acked-by: Jose Quaresma <jose.quaresma@foundries.io>
(From OE-Core rev: 14aa189c2e47a2c5a4a0099235a2605666651f74)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit beb2a76c591e985c6fc7ed473abd1bee27f955a2)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/gcc/gcc-common.inc

@@ -100,7 +100,7 @@ BINV = "${PV}"
 #S = "${WORKDIR}/gcc-${PV}"
 S = "${TMPDIR}/work-shared/gcc-${PV}-${PR}/gcc-${PV}"
 
-B = "${WORKDIR}/gcc-${PV}/build.${HOST_SYS}.${TARGET_SYS}"
+B ?= "${WORKDIR}/gcc-${PV}/build.${HOST_SYS}.${TARGET_SYS}"
 
 target_includedir ?= "${includedir}"
 target_libdir ?= "${libdir}"

meta/recipes-devtools/gcc/gcc-source.inc

@@ -18,6 +18,7 @@ INHIBIT_DEFAULT_DEPS = "1"
 DEPENDS = ""
 PACKAGES = ""
 
+B = "${WORKDIR}/build"
 
 # This needs to be Python to avoid lots of shell variables becoming dependencies.
 python do_preconfigure () {