cross-compilers: allow individual configuration

since configure supports only native configuration
a file 'cross-tcc.mak' needs to be created manually.
It is included in the Makefile if present.

# ----------------------------------------------------
# Example config-cross.mak:
#
# windows -> i386-linux cross-compiler
# (it expects the linux files in <prefix>/i386-linux)

ROOT-i386 = {B}/i386-linux
CRT-i386 = $(ROOT-i386)/usr/lib
LIB-i386 = $(ROOT-i386)/lib:$(ROOT-i386)/usr/lib
INC-i386 = {B}/lib/include:$(ROOT-i386)/usr/include
DEF-i386 += -D__linux__

# ----------------------------------------------------

Also:
- use libtcc1-<target>.a instead of directories
- add dummy arm assembler
- remove include dependencies from armeabi.c/lib-arm64.c
- tccelf/ld_add_file: add SYSROOT (when defined) to absolute
  filenames coming from ld-scripts

arm-gen.c

@@ -2147,3 +2147,7 @@ ST_FUNC void gen_vla_alloc(CType *type, int align) {
 /*************************************************************/
 #endif
 /*************************************************************/
+
+#ifndef TCC_IS_NATIVE
+#include "arm-asm.c"
+#endif