fix: code in non-executable ELF sections

tests/exec_section_in_asm.c

@@ -0,0 +1,28 @@
+/* Previously in TinyCC, ELF sections defined in assembly would always have the
+execute bit not set, so you would get segmentation faults when code in these
+sections was exectuted. This file is a minimal example of a file that will put
+the resulting code in a non-executable section (and invoke it) prior to the fix.
+*/
+#include <stdio.h>
+
+void *memset(void *dst, int c, int len);
+
+__asm__ (
+".section .text.nolibc_memset\n"
+".weak memset\n"
+"memset:\n"
+	"xchgl %eax, %esi\n\t"
+	"movq  %rdx, %rcx\n\t"
+	"pushq %rdi\n\t"
+	"rep stosb\n\t"
+	"popq  %rax\n\t"
+	"retq\n"
+);
+
+int main () {
+    char buf[10];
+    memset(&buf[0], 'A', 9);
+    buf[9] = 0;
+    puts(buf);
+    return 0;
+}

tests/exec_section_in_c.c

@@ -0,0 +1,13 @@
+/* Previously in TinyCC, ELF sections defined in attributes would always have
+the execute bit not set, so you would get segmentation faults when code in these
+sections was exectuted. This file is a minimal example of a file that will put
+the resulting code in a non-executable section (and invoke it) prior to the fix.
+*/
+__attribute__((section(".text.wumbo")))
+int wumbo (int arg) {
+    return arg * 2;
+}
+
+int main () {
+    return wumbo(2);
+}