arm: kirkwood: fix freeze on boot

arm: kirkwood: fix freeze on boot

Commit 6fe50e395080 ("arm: asm/system.h: mrc and mcr need .arm if
__thumb2__ is not set") is not a proper fix for the LTO link error
mentioned in its description. It causes 32-bit arm instructions to be
mixed with thumb instructions, which the Kirkwood SoCs do not support.
For example, board_init_r() is mostly generated in Thumb-1 mode as
expected since the build flags contain -mthumb -mthumb-interwork. The
MCR instruction corresponding to writefr_extra_feature_reg() is also
correcly emitted as a 32-bit ARM instruction (it cannot be encoded in
Thumb-1 anyways). The problem is, the compiler inlines the MCR without
generating the BX or BLX instruction which are needed to transition
between the ARM and the Thumb-1 states. From the objdump output:

006186a0 <board_init_r>:
board_init_r():
/home/jerome/work/u-boot/common/board_r.c:799
  6186a0:       b5f0            push    {r4, r5, r6, r7, lr}
  6186a2:       b0ab            sub     sp, #172        @ 0xac
get_gd():
/home/jerome/work/u-boot/./arch/arm/include/asm/global_data.h:127
  6186a4:       464a            mov     r2, r9
...
/home/jerome/work/u-boot/arch/arm/mach-kirkwood/cpu.c:242
  619aae:       9b15            ldr     r3, [sp, #84]   @ 0x54
writefr_extra_feature_reg():
/home/jerome/work/u-boot/./arch/arm/include/asm/arch/cpu.h:100
  619ab0:       ee2f3f11        mcr     15, 1, r3, cr15, cr1, {0}
                ^^^^^^^^
                32-bit ARM instruction

Further investigation is needed to understand how to fix the issue so
that the code size is minimal for all boards. In the mean time, this
fix disables LTO for the two problematic files (common/board_f.c and
common/board_r.c). This makes the Kirkwood-based boards bootable again.
The binary size is increased by 1048 bytes which is perfectly
acceptable.

Fixes: 6fe50e395080 ("arm: asm/system.h: mrc and mcr need .arm if __thumb2__ is not set")
Reported-by: Tony Dinh <mibodhi@gmail.com>
Tested-by: Tony Dinh <mibodhi@gmail.com>
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Tom Rini <trini@konsulko.com>