toolchain/gcc: correct the check expr for newer clang

This fixes gcc build error within clang 11.0, it tweaks the version
string from LLVM to clang.

Signed-off-by: Yorkie Liu <yorkiefixer@gmail.com>

toolchain/gcc/common.mk

@@ -89,7 +89,7 @@ endif
 
 GCC_CONFIGURE:= \
 	SHELL="$(BASH)" \
-	$(if $(shell gcc --version 2>&1 | grep LLVM), \
+	$(if $(shell gcc --version 2>&1 | grep -E "Apple.(LLVM|clang)"), \
 		CFLAGS="-O2 -fbracket-depth=512 -pipe" \
 		CXXFLAGS="-O2 -fbracket-depth=512 -pipe" \
 	) \