gdb/elfread.c: Enable ifunc support on ARM.

gdb/elfread.c: Enable ifunc support on ARM.

There are two failures in the gnu-ifunc.exp test on ARM. These are
due to the failure to resolve the correct target function when
attempting to breakpoint a GNU ifunc resolved function:

(gdb) break gnu_ifunc
Breakpoint 4 at gnu-indirect-function resolver at 0x2aacb5a2

when gnu_ifunc has been resolved this should actually be:

(gdb) break gnu_ifunc
Breakpoint 4 at 0x868c

There are two reasons for this. The first is that ARM does not have a
separate .got.plt section so looking this up will always fail. The second
is that the Thumb bit needs to be stripped from the address to allow
it to be reliably compared when inserting into the ifunc cache.

Tested with no regressions on arm-linux-gnueabihf and
x86_64-unknown-linux-gnu.

gdb/ChangeLog:

2014-02-10  Will Newton  <will.newton@linaro.org>

	* elfread.c (elf_rel_plt_read): Look for a .got section if
	looking up .got.plt fails.
	(elf_gnu_ifunc_resolve_by_got): Call gdbarch_addr_bits_remove
	on address passed to elf_gnu_ifunc_record_cache.
	(elf_gnu_ifunc_resolve_addr): Likewise.
	(elf_gnu_ifunc_resolver_return_stop): Likewise.