[libc] Implement setjmp/longjmp

Add a very simple setjmp/longjmp implementation. No destructors or other
cleanup is handled.

src/libraries/libc/arch/amd64/crt/crt0.s

@@ -1,3 +1,10 @@
+; This file is part of the C standard library for the jsix operating
+; system.
+;
+; This Source Code Form is subject to the terms of the Mozilla Public
+; License, v. 2.0. If a copy of the MPL was not distributed with this
+; file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
 extern _GLOBAL_OFFSET_TABLE_
 
 extern main

src/libraries/libc/include/setjmp.h

@@ -10,4 +10,10 @@
   * file, You can obtain one at https://mozilla.org/MPL/2.0/.
   */
 
-#error setjmp.h is not yet implemented.
+#include <stdint.h>
+#include <stdnoreturn.h>
+
+typedef uint64_t jmp_buf[9];
+
+int setjmp(jmp_buf env);
+_Noreturn void longjmp(jmp_buf env, int val);

src/libraries/libc/setjmp/setjmp.s

@@ -0,0 +1,76 @@
+; This file is part of the C standard library for the jsix operating
+; system.
+;
+; This Source Code Form is subject to the terms of the Mozilla Public
+; License, v. 2.0. If a copy of the MPL was not distributed with this
+; file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+struc JMPBUF
+.rbx:       resq 1
+.rsp:       resq 1
+.rbp:       resq 1
+.r12:       resq 1
+.r13:       resq 1
+.r14:       resq 1
+.r15:       resq 1
+
+.retaddr:   resq 1
+
+.mxcsr:     resd 1
+.fcw:       resw 1
+
+endstruc
+
+global setjmp: function (setjmp.end - setjmp)
+setjmp:
+    push rbp
+    mov rbp, rsp
+
+    mov rax, [rsp + 8]
+    mov [rdi + JMPBUF.retaddr], rax
+
+    mov [rdi + JMPBUF.rbx], rbx
+    mov [rdi + JMPBUF.rsp], rsp
+    mov [rdi + JMPBUF.rbp], rbp
+    mov [rdi + JMPBUF.r12], r12
+    mov [rdi + JMPBUF.r13], r13
+    mov [rdi + JMPBUF.r14], r14
+    mov [rdi + JMPBUF.r15], r15
+
+    stmxcsr [rdi + JMPBUF.mxcsr]
+    fnstcw [rdi + JMPBUF.fcw]
+
+    mov rax, 0 ; actual setjmp returns 0
+
+    pop rbp
+    ret
+.end:
+
+
+global longjmp: function (longjmp.end - longjmp)
+longjmp:
+    push rbp
+    mov rbp, rsp
+
+    mov rbx, [rdi + JMPBUF.rbx]
+    mov rbp, [rdi + JMPBUF.rbp]
+    mov r12, [rdi + JMPBUF.r12]
+    mov r13, [rdi + JMPBUF.r13]
+    mov r14, [rdi + JMPBUF.r14]
+    mov r15, [rdi + JMPBUF.r15]
+
+    ldmxcsr [rdi + JMPBUF.mxcsr]
+    fnclex
+    fldcw [rdi + JMPBUF.fcw]
+
+    mov rsp, [rdi + JMPBUF.rsp]
+
+    mov rax, rsi
+    cmp rax, 0
+    jne .done
+    mov rax, 1
+
+.done:
+    pop rbp
+    ret
+.end: