nintendoswitch: Add experimental Nintendo Switch support without CRT

Bare minimal nintendo switch support using LLD

targets/nintendoswitch.json

@@ -0,0 +1,28 @@
+{
+  "llvm-target": "aarch64",
+  "build-tags": ["nintendoswitch", "arm64"],
+  "goos": "linux",
+  "goarch": "arm64",
+  "compiler": "clang",
+  "linker": "ld.lld",
+  "rtlib": "compiler-rt",
+  "libc": "picolibc",
+  "gc": "conservative",
+  "relocation-model": "pic",
+  "cflags": [
+    "-target", "aarch64-none-linux-gnu",
+    "-mtune=cortex-a57",
+    "-fPIE",
+    "-Werror",
+    "-Qunused-arguments",
+    "-fshort-enums",
+    "-fomit-frame-pointer",
+    "-fno-exceptions", "-fno-unwind-tables",
+    "-ffunction-sections", "-fdata-sections"
+  ],
+  "linkerscript": "targets/nintendoswitch.ld",
+  "extra-files": [
+    "targets/nintendoswitch.s",
+    "src/runtime/runtime_nintendoswitch.s"
+  ]
+}

targets/nintendoswitch.ld

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+OUTPUT_FORMAT(elf64-littleaarch64)
+OUTPUT_ARCH(aarch64)
+ENTRY(_start)
+
+PHDRS
+{
+  text    PT_LOAD     FLAGS(5);
+  rodata  PT_LOAD     FLAGS(4);
+  data    PT_LOAD     FLAGS(6);
+  bss     PT_LOAD     FLAGS(6);
+  dynamic PT_DYNAMIC;
+}
+
+SECTIONS
+{
+  . = 0;
+
+  .text : ALIGN(0x1000) {
+    HIDDEN(__text_start = .);
+    KEEP(*(.text.jmp))
+
+    . = 0x80;
+
+    *(.text .text.*)
+  }
+
+  /* Read-only sections */
+
+  . = ALIGN(0x1000);
+
+  .rodata : { *(.rodata .rodata.*) } :rodata
+  .mod0 : {
+    KEEP(crt0.nso.o(.data.mod0))
+    KEEP(crt0.nro.o(.data.mod0))
+    KEEP(crt0.lib.nro.o(.data.mod0))
+  }
+
+  /* Read-write sections */
+   . = ALIGN(0x1000);
+
+  .data : {
+    *(.data .data.*)
+  } :data
+
+  .dynamic : {
+    HIDDEN(__dynamic_start = .);
+    *(.dynamic)
+  }
+
+  /* BSS section */
+
+  . = ALIGN(0x1000);
+
+  .bss : {
+    HIDDEN(__bss_start = .);
+    *(.bss .bss.*)
+    *(COMMON)
+    . = ALIGN(8);
+    HIDDEN(__bss_end = .);
+  } :bss
+}

targets/nintendoswitch.s

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+.section .text.jmp, "x"
+.global _start
+_start:
+    b start
+    .word _mod_header - _start
+
+.section .data.mod0
+    .word 0, 8
+
+.global _mod_header
+_mod_header:
+    .ascii "MOD0"
+    .word __dynamic_start - _mod_header
+    .word __bss_start - _mod_header
+    .word __bss_end - _mod_header
+    .word 0, 0 // eh_frame_hdr start/end
+    .word 0 // runtime-generated module object offset
+
+.section .text, "x"
+.global start
+start:
+
+    // save lr
+    mov  x7, x30
+
+    // get aslr base
+    bl   +4
+    sub  x6, x30, #0x88
+
+    // context ptr and main thread handle
+    mov  x5, x0
+    mov  x4, x1
+
+    // clear .bss
+    adrp x5, __bss_start
+    add x5, x5, #:lo12:__bss_start
+    adrp x6, __bss_end
+    add x6, x6, #:lo12:__bss_end
+
+bssloop:
+    cmp x5, x6
+    b.eq run
+    str xzr, [x5]
+    add x5, x5, 8
+    b bssloop
+
+run:
+    // call entrypoint
+    adrp x30, exit
+    add  x30, x30, #:lo12:exit
+    b    main