Skip 'bx reg' trampoline on arm-none-eabi

After this patch
<https://gcc.gnu.org/ml/gcc-patches/2005-01/msg00813.html> applied to
GCC, a new trampoline is generated but GDB doesn't recognize it.  This
patch is to teach GDB to understand this trampoline.  See details
about this trampoline and the heuristics in the comments.

gdb:

2014-06-24  Yao Qi  <yao@codesourcery.com>

	* arm-tdep.c (arm_skip_bx_reg): New function.
	(arm_skip_stub): Call arm_skip_bx_reg.
Apply stub unwinder to 'bx reg' trampoline

In target arm-none-eabi, prologue unwinder is used for trampoline
'bx reg'.  However, in target arm-linux, exidx unwinder is selected for
trampoline at first, which is not expected.  The main function and the
trampoline is,

   0x00009dfc <main+0>: push    {r4, r5, r6, r7, lr}
   ......
   0x0000ac30 <main+3636>:      ldrdeq  r3, [r1], -r8
   0x0000ac34:  bx      r2
   0x0000ac36:  bx      r4

and .ARM.exidx is:

0x9dfc <main>: @0xb404
  Compact model index: 1
  0x97      vsp = r7
  0x20      vsp = vsp + 132
  0x3f      vsp = vsp + 256
  0x80 0xf0 pop {r8, r9, r10, r11}
  0xab      pop {r4, r5, r6, r7, r14}

0xac38 <__aeabi_drsub>: 0x1 [cantunwind]

Trampolines 'bx r2' and 'bx r4' doesn't belong to main, but the exidx
for main is still selected form them because there is no end address
of each exidx entry.

Instead of teaching exidx unwinder ignore this trampoline (which looks
complicated and error prone), I decide to let stub unwinder to handle
trampoline, because stub undwinder is installed before exidx unwinder,
and this trampoline can be regarded as a stub too.

This patch is to add the code to match 'bx reg' trampoline in the
sniffer of stub unwinder.

gdb:

2014-06-24  Yao Qi  <yao@codesourcery.com>

	* arm-tdep.c (arm_stub_unwind_sniffer): Return 1 if
	arm_skip_bx_reg returns	non-zero.
Skip 'bx reg' on arm-linux

In arm-tdep.c, arm_skip_stub is installed to gdbarch
skip_trampoline_code, but in arm-linux-tdep.c,
find_solib_trampoline_target is installed to skip_trampoline_code.
That means gdb configured for arm-linux target doesn't recognize some
arm specific trampolines or stubs.  Beside handling generic solib
trampoline, gdb for arm-linux target should be able to handle arm
specific trampolines.  This patch is to skip arm specific stubs, if
any, and as a fallback, skip the generic solib trampoline.

gdb:

2014-06-24  Yao Qi  <yao@codesourcery.com>

	* arm-linux-tdep.c (arm_linux_skip_trampoline_code): New.
	(arm_linux_init_abi): Set skip_trampoline_code with
	gdbarch_skip_trampoline_code instead of
	find_solib_trampoline_target.
Don't strip section defining _SDA_BASE_

Prior to 93d1b056 _SDA_BASE_ used to be defined in a linker script
output section.  Now _SDA_BASE_ is defined in an input section that is
subject to being stripped.  If the section is stripped we don't output
the symbol, which results in --emit-relocs trying to emit relocs with
dangling references to _SDA_BASE_.

	* elf32-ppc.c (ppc_elf_size_dynamic_sections): Arrange to keep
	.sdata/.sdata2 when _SDA_BASE_/_SDA2_BASE_ should be output
	for --emit-relocs.
Minor improvements in manual indexing.

	* doc/gdb.texinfo (Screen Size): Add more index entries.
Fix dwp to allow intermediate .dwp files with no .debug_types sections.

gold/
	* dwp.cc (Dwo_file::read): Allow files with no .debug_types
	sections.
	(Dwo_file::sized_read_unit_index): Likewise.
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