[LibOS, Pal] add .file directive to .S file to make debug easier

gas includes dwarf info about compilation unit into .o compiled
from .S. Hoever, without .file directive, gnu linker drops dwarf
compilation unit info about .S. from .so (libpal.so libsysdb.so).
As result gdb fails to open .S when debugging .S file.
Add .file directive to .S files to make debug .S easier.

My ld version is
> $ ld --version
> GNU ld (GNU Binutils for Ubuntu) 2.30

Signed-off-by: Isaku Yamahata <isaku.yamahata@gmail.com>

LibOS/shim/src/start.S

@@ -16,6 +16,7 @@
    ...
    NULL
  */
+    .file "start.S"
     .text
     .globl shim_start
     .type shim_start,@function

LibOS/shim/src/syscallas.S

@@ -24,6 +24,7 @@
 
 #include "asm-offsets.h"
 
+        .file "syscallas.S"
         .global syscalldb
         .type syscalldb, @function
         .extern shim_table, debug_unsupp

LibOS/shim/src/vdso/vdso-data.S

@@ -1,3 +1,4 @@
+	.file "vdso-data.S"
 	.section .rodata
 
 	.global vdso_so

LibOS/shim/src/vdso/vdso-note.S

@@ -1,3 +1,4 @@
+	.file "vdso-note.S"
 	/* This .note section informs dynamic linker about vDSO */
 	.section .note.Linux, "a", @note
 

Pal/src/host/Linux-SGX/enclave_entry.S

@@ -1,6 +1,8 @@
 #include "sgx_arch.h"
 #include "asm-offsets.h"
 
+	.file "enclave_entry.S"
+
 # In some cases, like bogus parameters passed to enclave_entry, it's tricky to
 # return cleanly (passing the correct return address to EEXIT, OCALL_EXIT can
 # be interrupted, etc.). Since those cases should only ever happen with a

Pal/src/host/Linux-SGX/sgx_entry.S

@@ -3,6 +3,8 @@
 
 #include "asm-offsets.h"
 
+	.file "sgx_entry.S"
+
 	.extern tcs_base
 
 	.global sgx_ecall

Pal/src/host/Linux/clone-x86_64.S

@@ -30,6 +30,8 @@
 
 #include "sysdep-x86_64.h"
 
+	.file "clone-x86_64.S"
+
 #define CLONE_VM	0x00000100
 #define CLONE_THREAD	0x00010000
 

Pal/src/host/Linux/gettimeofday-x86_64.S

@@ -26,6 +26,8 @@
 
 #include "sysdep-x86_64.h"
 
+	.file "gettimeofday-x86_64.S"
+
 /* For the calculation see asm/vsyscall.h.  */
 #define VSYSCALL_ADDR_vgettimeofday	0xffffffffff600000
 

Pal/src/user_start.S

@@ -17,6 +17,7 @@
 						NULL
 */
 
+	.file "user_start.S"
 	.text
 	.globl _start
 	.type _start,@function