GCC toolchain: Add {SYSROOT} substitution with support for whitespaces

Any instance of {SYROOT} string in config's CFLAGS and LDFLAGS is replaced by sysroot path obtained from "gcc -print-sysroot"

<?xml version="1.0" encoding="UTF-8"?>
<BeremizRoot URI_location="LOCAL://">
  <TargetType/>
</BeremizRoot>