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

libtcc.c

@@ -1120,7 +1120,6 @@ static int tcc_add_library_internal(TCCState *s, const char *fmt,
     return -1;
 }
 
-#ifndef TCC_TARGET_PE
 /* find and load a dll. Return non zero if not found */
 /* XXX: add '-rpath' option support ? */
 ST_FUNC int tcc_add_dll(TCCState *s, const char *filename, int flags)
@@ -1128,7 +1127,6 @@ ST_FUNC int tcc_add_dll(TCCState *s, const char *filename, int flags)
     return tcc_add_library_internal(s, "%s/%s", filename, flags,
         s->library_paths, s->nb_library_paths);
 }
-#endif
 
 ST_FUNC int tcc_add_crt(TCCState *s, const char *filename)
 {