Do not skip prologue for asm (.S) files

Do not skip prologue for asm (.S) files

GDB tries to skip prologue for .S files according to .debug_line but it then
places the breakpoint to a location where it is never hit.

This is because #defines in .S files cause prologue skipping which is
completely inappropriate, for s390x:

glibc/sysdeps/unix/syscall-template.S
78:/* This is a "normal" system call stub: if there is an error,
79:   it returns -1 and sets errno.  */
80:
81:T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
82:     ret

00000000000f4210 T __select
 Line Number Statements:
  Extended opcode 2: set Address to 0xf41c8
  Advance Line by 80 to 81
  Copy
  Advance PC by 102 to 0xf422e
  Special opcode 6: advance Address by 0 to 0xf422e and Line by 1 to 82
  Special opcode 34: advance Address by 2 to 0xf4230 and Line by 1 to 83
  Advance PC by 38 to 0xf4256
  Extended opcode 1: End of Sequence
  Compilation Unit @ offset 0x28b3e0:
 <0><28b3eb>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <28b3ec>   DW_AT_stmt_list   : 0x7b439
    <28b3f0>   DW_AT_low_pc	 : 0xf41c8
    <28b3f8>   DW_AT_high_pc     : 0xf4256
    <28b400>   DW_AT_name        : ../sysdeps/unix/syscall-template.S
    <28b423>   DW_AT_comp_dir    : /usr/src/debug////////glibc-2.17-c758a686/misc
    <28b452>   DW_AT_producer    : GNU AS 2.23.52.0.1
    <28b465>   DW_AT_language    : 32769        (MIPS assembler)

without debuginfo or with debuginfo and the fix - correct address:
(gdb) b select
Breakpoint 1 at 0xf4210

It is also where .dynsym+.symtab point to:
00000000000f4210 T __select
00000000000f4210 W select

with debuginfo, without the fix:
(gdb) b select
Breakpoint 1 at 0xf41c8: file ../sysdeps/unix/syscall-template.S, line 81.

One part is to behave for asm files similar way like for 'locations_valid':
  /* Symtab has been compiled with both optimizations and debug info so that
     GDB may stop skipping prologues as variables locations are valid already
     at function entry points.  */
  unsigned int locations_valid : 1;

The other part is to extend the 'locations_valid'-like functionality more.

Both minsym_found and find_function_start_sal need to be patched, otherwise
their addresses do not match and GDB regresses on ppc64:

gdb/ChangeLog
2015-06-26  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* linespec.c (minsym_found): Reset sal.PC for COMPUNIT_LOCATIONS_VALID
	and language_asm..
	* symtab.c (find_function_start_sal): Likewise.

gdb/testsuite/ChangeLog
2015-06-26  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.arch/amd64-prologue-skip.S: New file.
	* gdb.arch/amd64-prologue-skip.exp: New file.