Fixed to allow CFLAGS to accumulate for building host libbfd

In this case CFLAGS was set to "-Wno-error" before building host
libbfd, and it's followed by -Werror to invoke CC.  There are a
few warnings treated as error but -Wno-error suppress them.

Resetting CFLAGS to -m32 breaks the build

Changed it to accumulate CFLAGS rather than resetting CFLAGS

Change-Id: I118a848c27f91407b75beabea10c35883ca16e5a