Do arm_abi detection for ELFOSABI_GNU binaries

Do arm_abi detection for ELFOSABI_GNU binaries

On ARM systems, gdb must determine which style of breakpoint to use
(see the comments at the beginning of gdb/arm-linux-tdep.c).  In
arm_gdbarch_init we only attempt to extract the eabi version from the
ELF binary if it is a ELFOSABI_NONE binary.  If the binary is
ELFOSABI_GNU instead, we end up defaulting to the old style OABI
syscall breakpoint instruction.  On a Linux kernel built without
CONFIG_OABI_COMPAT, this triggers a SIGILL in ld when attempting to
execute any ELFOSABI_GNU program.
(e.g. https://github.com/raspberrypi/linux/issues/766)

gdb/

2015-05-28  Kyle Huey  <me@kylehuey.com>  (tiny patch)

	* gdb/arm-tdep.c (arm_gdbarch_init): Perform arm_abi detection on
	ELFOSABI_GNU binaries.