BaseTools: Update antlr makefile to use cc by default

Update the antlr makefile to remove the explicit setting of CC to either
clang or gcc. This causes it to use /usr/bin/cc or whatever the user has
set $(CC) to.

This removes the last dependency on gcc for BaseTools.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>