ARMv7-R: fix CPSR corruption after exception handling

A sporadic hang with consequent crash is observed when booting:

    arm_prefetchabort: Prefetch abort. PC: 04d34a00 IFAR: 04d34a00 IFSR: 00000008
    up_assert: Assertion failed at file:armv7-r/arm_prefetchabort.c line: 87 task: init
    up_dumpstate: Current sp: 004c3df0
    up_dumpstate: Interrupt stack:
    up_dumpstate:   base: 004c05fc
    up_dumpstate:   size: 00000800
    up_dumpstate: User stack:
    up_dumpstate:   base: 004c3f58
    up_dumpstate:   size: 00000fec
    up_dumpstate: User Stack
    up_stackdump: 004c3de0: 004a0d14 004c3df0 004c3f58 004a0d20 00000057 004c2c58 09000000 004a42a4
    up_stackdump: 004c3e00: 00000003 004c3e10 004a0f1c 004bbcef 33c44b00 004a0f28 04d34a00 00000008
    up_stackdump: 004c3e20: 00000008 004a01bc 004bfd38 00000001 00007fff 00000001 34134a00 d83e4c00
    up_stackdump: 004c3e40: 09000000 00000000 33c44b00 d83e4c00 0c3f4c00 00000000 00000000 00000003
    up_stackdump: 004c3e60: 004c3e70 004a1494 04d34a00 200b0253 004c3ed8 004a5298 004c3ed8 6d00006d
    up_stackdump: 004c3e80: 0000006d 004bc3f4 00000009 004a4f64 00000009 b9e0784f 333f3ed0 69d4227d
    up_stackdump: 004c3ea0: d81f09bd 0f867344 5a7e2c12 8acefd34 5d00dc1b 004bc432 004c3f08 004c0e08
    up_stackdump: 004c3ec0: 00000000 00000000 00000000 00000000 00000000 004a4210 004a5258 004a5300
    up_stackdump: 004c3ee0: 00000000 00000001 ffffffff 004c3f80 000002b0 004a4234 00000007 004c3f08
    up_stackdump: 004c3f00: 004a58b4 004bc432 004bc3f4 004c3f80 000002b0 0000029c 00000000 0000029c
    up_stackdump: 004c3f20: 00000000 004a5900 0000ff01 00000000 00000000 004a61f4 00000000 004a5fa4
    up_stackdump: 004c3f40: 00000000 004a5f6c 00000000 004a2668 00000000 00000000 b7509f04 004c3f64
    up_registerdump: R0: 00000001 00007fff 00000001 34134a00 d83e4c00 09000000 00000000 33c44b00
    up_registerdump: R8: d83e4c00 0c3f4c00 00000000 00000000 00000003 004c3e70 004a1494 04d34a00
    up_registerdump: CPSR: 200b0253

It seems to be caused by the corrupted or wrong CPSR restored on return
from exception. NuttX restores the context using code like this:

    msr spsr, r1

GCC translates this to:

    msr spsr_fc, r1

As a result, not all SPSR fields are updated on exception return. This
should be:

    msr spsr_fsxc, r1

On some evaluation boards, spsr_svc may have totally invalid value at
power-on-reset. As it is not initialized at boot, the code above may
result in the corruption of cpsr and thus unexpected behavior.

Reported-by: Eunbong Song <eunb.song@samsung.com>
Signed-off-by: Heesub Shin <heesub.shin@samsung.com>