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

lib/alloca-arm.S

@@ -3,8 +3,15 @@
 	.global	alloca
 	.type	alloca, %function
 alloca:
+#ifdef __TINYC__
+        .int 0xe060d00d
+        .int 0xe3cdd007
+        .int 0xe1a0000d
+        .int 0xe1a0f00e
+#else
 	rsb	sp, r0, sp
 	bic	sp, sp, #7
 	mov	r0, sp
 	mov	pc, lr
+#endif
 	.size	alloca, .-alloca