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'?>
<OPCUAClient xmlns:xsd="http://www.w3.org/2001/XMLSchema"/>