Makefile: Set HOSTCC as gcc or cc respectively

The HOSTCC should be set in .xcompile, which tests the existence of gcc
and cc. But the .xcompile has to be included after kconfig/Makefile. So
building util/kconfig uses the seperated HOSTCC definition above it,
instead of the one in .xcompile.

For the system which clang is the default host compiler, gcc is not
installed by default. In that case, we need to set HOSTCC as cc.

Change-Id: I1e51a37c4426e2c97d36a31f26a18ab4b0d0608d
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/12331
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

Makefile

@@ -79,7 +79,7 @@ endif
 # Disable implicit/built-in rules to make Makefile errors fail fast.
 .SUFFIXES:
 
-HOSTCC := gcc
+HOSTCC := $(if $(shell type gcc 2>/dev/null), gcc, cc)
 HOSTCXX = g++
 HOSTCFLAGS := -g
 HOSTCXXFLAGS := -g