GCC-specific macros
Now compiling with SECD interpreter

Changes:
0) added SECD codes interpreter + simple Lisp interpreter
More about SECD machine: https://github.com/EarlGray/SECD
The lisp semantics is quite close to PicoLisp, but its syntax is a
small subset of Scheme.
SECD code interpreter: core/secd.c
Scheme interpreter in SECD: core/scm2secd.secd
+ Makefile changes to embed .secd in the ELF kernel binary

1) additional defines (like "COSEC") in include/globl.h
`bool` changed to `enum { false, true }` from `char`;

2) added `isdigit` and `isspace` routines;

3) added dummy emulation of some stdlib.h file operations
TODO: implement ELF symbols search