Try to implement linux si_uc.inc for xtensa CPU

rtl/linux/Makefile

@@ -410,7 +410,7 @@ ifeq ($(ARCH),sparc64)
 endif
 ifeq ($(ARCH),xtensa)
 override LOADERS=
-SYSINIT_UNITS=si_prc si_dll si_c
+SYSINIT_UNITS=si_prc si_dll si_c si_uc
 endif
 ifeq ($(ARCH),loongarch64)
 override LOADERS=

rtl/linux/Makefile.fpc

@@ -135,7 +135,7 @@ endif
 
 ifeq ($(ARCH),xtensa)
 override LOADERS=
-SYSINIT_UNITS=si_prc si_dll si_c
+SYSINIT_UNITS=si_prc si_dll si_c si_uc
 endif
 
 ifeq ($(ARCH),loongarch64)

rtl/linux/xtensa/si_uc.inc

@@ -0,0 +1,112 @@
+{
+    This file is part of the Free Pascal run time library.
+    Copyright (c) 2019 by Jeppe Johansen.
+
+    See the file COPYING.FPC, included in this distribution,
+    for details about the copyright.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+
+ **********************************************************************}
+
+{******************************************************************************
+                          Process start/halt
+ ******************************************************************************}
+
+var
+  dlexitproc : pointer;
+
+var
+  BSS_START: record end; external name '__bss_start';
+  STACK_PTR: record end; external name '__stkptr';
+
+  uclibc_init : TProcedure; external name '_init';
+  uclibc_fini : TProcedure; external name '_fini';
+  procedure uclibc_exit(e : longint); weakexternal name 'exit';
+  procedure uclibc_main(main : TProcedure; argc : longint; argv : ppchar; init : TProcedure;  fini : TProcedure; sp : pointer); external name '__uClibc_main';
+
+procedure _FPC_xtensa_enter(at_exit: TProcedure; sp: pptruint);
+  var
+    argc: ptruint;
+    argv: ppchar;
+  begin
+    argc:=sp[0];
+    argv:=@sp[1];
+
+    initialstkptr:=sp;
+    operatingsystem_parameter_argc:=argc;
+    operatingsystem_parameter_argv:=argv;
+    operatingsystem_parameter_envp:=@sp[argc+2];
+ {$ifdef FPC_ABI_WINDOWED }
+    { Windowed ABI }
+    asm
+      movi a0,0
+      movi a6,PascalMain
+      addi a8,argv
+      addi a7,a8,-4
+      movi a9,uclibc_init
+      movi a10,uclibc_fini
+      addi a11,at_exit
+      movi a4,uclibc_main
+      s32i a1,a1,0
+      callx4 a4
+      ill
+    end;
+ {$endif}
+ {$ifdef FPC_ABI_CALL0 }
+    { Call0 ABI}
+    asm
+      movi a7,at_exit
+      movi a2,PascalMain
+      addi a4,argv
+      addi a3,a4,-4
+      movi a5,uclibc_init
+      movi a6,uclibc_fini
+      s32i a1,a1,0
+      movi a0,uclibc_main
+      callx0 a0
+      ill
+    end;
+ {$endif}
+  end;
+
+
+procedure _FPC_proc_start; assembler; public name '_start';
+  asm
+    ill
+  end;
+
+
+procedure _FPC_xtensa_exit(e:longint); assembler;
+  asm
+    mov a6,a3
+    movi a2,119
+    syscall
+  end;
+
+
+procedure _FPC_proc_haltproc(e:longint); cdecl; public name '_haltproc';
+  begin
+    while true do
+      begin
+ {$ifdef FPC_ABI_WINDOWED }
+    { Windowed ABI }
+        asm
+	  l32i a5,e
+	  movi a4,uclibc_exit
+	  callx4 a4
+	end;
+ {$endif}
+ {$ifdef FPC_ABI_CALL0 }
+    { Call0 ABI}
+        asm
+	  l32i a2,e
+	  movi a0,uclibc_exit
+	  callx0 a0
+	end;
+ {$endif}
+        _FPC_xtensa_exit(e);
+      end;
+  end;