BaseTools: Replace ARCH with HOST_ARCH in C Makefile to avoid conflict

https://bugzilla.tianocore.org/show_bug.cgi?id=793

ARCH is too generic. It may cause confuse of target arch or host arch.
To be clarified, replace it with HOST_ARCH in BaseTools C Makefile.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>

BaseTools/Source/C/GnuGenBootSector/GNUmakefile

@@ -10,7 +10,7 @@
 # THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
 # WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
 #
-ARCH ?= IA32
+HOST_ARCH ?= IA32
 MAKEROOT ?= ..
 
 APPNAME = GnuGenBootSector