Find the .text segment when it isn't first in the binary

Currently, LD assumes that the first PT_LOAD segment in the binary is the .text.
This used to be the case in older compilers (and is usually the case), but the
latest nightly version of rust actually puts a RO segment before the .text
segment, containing various misc metadata.

This patch properly changes the method of finding the .text by looking for
PT_LOAD segments that are executable.

Author: roblabla

unwind/src/find_cfi/ld.rs

@@ -40,6 +40,7 @@ struct Phdr64 {
 
 const PT_GNU_EH_FRAME: u32 = 0x6474e550;
 const PT_LOAD: u32 = 1;
+const PF_X: u32 = 1;
 
 type PhdrCb = extern "C" fn(info: *const DlPhdrInfo, size: usize, data: *mut c_void) -> c_int;
 extern "C" {
@@ -56,7 +57,7 @@ extern "C" fn callback(info: *const DlPhdrInfo, size: usize, data: *mut c_void)
 
         let phdr = slice::from_raw_parts((*info).phdr, (*info).phnum as usize);
 
-        if let Some(text) = phdr.iter().filter(|x| x.type_ == PT_LOAD).next() {
+        if let Some(text) = phdr.iter().filter(|x| x.type_ == PT_LOAD && x.flags & PF_X != 0).next() {
             if let Some(eh_frame) = phdr.iter().filter(|x| x.type_ == PT_GNU_EH_FRAME).next() {
                 let start_addr = (*info).addr + text.vaddr;
                 let cfi_start = (*info).addr + eh_frame.vaddr;