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-asm.c

@@ -0,0 +1,83 @@
+/*************************************************************/
+/*
+ *  ARM dummy assembler for TCC
+ *
+ */
+
+#ifdef TARGET_DEFS_ONLY
+
+#define CONFIG_TCC_ASM
+#define NB_ASM_REGS 16
+
+ST_FUNC void g(int c);
+ST_FUNC void gen_le16(int c);
+ST_FUNC void gen_le32(int c);
+
+/*************************************************************/
+#else
+/*************************************************************/
+
+/* XXX: make it faster ? */
+ST_FUNC void g(int c)
+{
+    int ind1;
+    if (nocode_wanted)
+        return;
+    ind1 = ind + 1;
+    if (ind1 > cur_text_section->data_allocated)
+        section_realloc(cur_text_section, ind1);
+    cur_text_section->data[ind] = c;
+    ind = ind1;
+}
+
+ST_FUNC void gen_le16 (int i)
+{
+    g(i);
+    g(i>>8);
+}
+
+ST_FUNC void gen_le32 (int i)
+{
+    gen_le16(i);
+    gen_le16(i>>16);
+}
+
+ST_FUNC void gen_expr32(ExprValue *pe)
+{
+    gen_le32(pe->v);
+}
+
+ST_FUNC void asm_opcode(TCCState *s1, int opcode)
+{
+}
+
+ST_FUNC void subst_asm_operand(CString *add_str, SValue *sv, int modifier)
+{
+}
+
+/* generate prolog and epilog code for asm statement */
+ST_FUNC void asm_gen_code(ASMOperand *operands, int nb_operands,
+                         int nb_outputs, int is_output,
+                         uint8_t *clobber_regs,
+                         int out_reg)
+{
+}
+
+ST_FUNC void asm_compute_constraints(ASMOperand *operands,
+                                    int nb_operands, int nb_outputs,
+                                    const uint8_t *clobber_regs,
+                                    int *pout_reg)
+{
+}
+
+ST_FUNC void asm_clobber(uint8_t *clobber_regs, const char *str)
+{
+}
+
+ST_FUNC int asm_parse_regvar (int t)
+{
+    return -1;
+}
+
+/*************************************************************/
+#endif /* ndef TARGET_DEFS_ONLY */