Revert "config: don't hardcode toolchain binaries"

This reverts commit b41666f2c1.  This breaks
various OSSTest builds.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

config/StdGNU.mk

@@ -1,27 +1,27 @@
-AS        ?= $(CROSS_COMPILE)as
-LD        ?= $(CROSS_COMPILE)ld
+AS         = $(CROSS_COMPILE)as
+LD         = $(CROSS_COMPILE)ld
 ifeq ($(clang),y)
-CC        ?= $(CROSS_COMPILE)clang
-CXX       ?= $(CROSS_COMPILE)clang++
-LD_LTO    ?= $(CROSS_COMPILE)llvm-ld
+CC         = $(CROSS_COMPILE)clang
+CXX        = $(CROSS_COMPILE)clang++
+LD_LTO     = $(CROSS_COMPILE)llvm-ld
 else
-CC        ?= $(CROSS_COMPILE)gcc
-CXX       ?= $(CROSS_COMPILE)g++
-LD_LTO    ?= $(CROSS_COMPILE)ld
+CC         = $(CROSS_COMPILE)gcc
+CXX        = $(CROSS_COMPILE)g++
+LD_LTO     = $(CROSS_COMPILE)ld
 endif
-CPP       ?= $(CC) -E
-AR        ?= $(CROSS_COMPILE)ar
-RANLIB    ?= $(CROSS_COMPILE)ranlib
-NM        ?= $(CROSS_COMPILE)nm
-STRIP     ?= $(CROSS_COMPILE)strip
-OBJCOPY   ?= $(CROSS_COMPILE)objcopy
-OBJDUMP   ?= $(CROSS_COMPILE)objdump
-SIZEUTIL  ?= $(CROSS_COMPILE)size
+CPP        = $(CC) -E
+AR         = $(CROSS_COMPILE)ar
+RANLIB     = $(CROSS_COMPILE)ranlib
+NM         = $(CROSS_COMPILE)nm
+STRIP      = $(CROSS_COMPILE)strip
+OBJCOPY    = $(CROSS_COMPILE)objcopy
+OBJDUMP    = $(CROSS_COMPILE)objdump
+SIZEUTIL   = $(CROSS_COMPILE)size
 
 # Allow git to be wrappered in the environment
 GIT        ?= git
 
-INSTALL     ?= install
+INSTALL      = install
 INSTALL_DIR  = $(INSTALL) -d -m0755 -p
 INSTALL_DATA = $(INSTALL) -m0644 -p
 INSTALL_PROG = $(INSTALL) -m0755 -p