Leave 'linux' defined on x86/x86-64, like arm.

Change-Id: I53013a45c19a643a985de3f339b8b3f8f4dc39d2
diff --git a/core/combo/TARGET_linux-x86.mk b/core/combo/TARGET_linux-x86.mk
index 1361be2..01f8242 100644
--- a/core/combo/TARGET_linux-x86.mk
+++ b/core/combo/TARGET_linux-x86.mk
@@ -106,7 +106,6 @@
 
 $(combo_2nd_arch_prefix)TARGET_GLOBAL_CFLAGS += \
 			-O2 \
-			-Ulinux \
 			-Wa,--noexecstack \
 			-Werror=format-security \
 			-D_FORTIFY_SOURCE=2 \
diff --git a/core/combo/TARGET_linux-x86_64.mk b/core/combo/TARGET_linux-x86_64.mk
index 2e06263..a5a745b 100644
--- a/core/combo/TARGET_linux-x86_64.mk
+++ b/core/combo/TARGET_linux-x86_64.mk
@@ -106,7 +106,6 @@
 
 TARGET_GLOBAL_CFLAGS += \
 			-O2 \
-			-Ulinux \
 			-Wa,--noexecstack \
 			-Werror=format-security \
 			-D_FORTIFY_SOURCE=2 \