selftests/powerpc: Add FPU denormal test

Add a testcase that tries to trigger the FPU denormal exception on
Power8 or earlier CPUs.

Prior to commit 4557ac6b344b ("powerpc/64s/exception: Fix 0x1500
interrupt handler crash") this would trigger a crash such as:

Oops: Exception in kernel mode, sig: 5 [#1]
LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA PowerNV
Modules linked in: iptable_mangle xt_MASQUERADE iptable_nat nf_nat xt_conntrack nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ipt_REJECT nf_reject_ipv4 xt_tcpudp tun bridge stp llc ip6table_filter ip6_tables iptable_filter fuse kvm_hv binfmt_misc squashfs mlx4_ib ib_uverbs dm_multipath scsi_dh_rdac scsi_dh_alua ib_core mlx4_en sr_mod cdrom bnx2x lpfc mlx4_core crc_t10dif scsi_transport_fc sg mdio vmx_crypto crct10dif_vpmsum leds_powernv powernv_rng rng_core led_class powernv_op_panel sunrpc ip_tables x_tables autofs4
CPU: 159 PID: 6854 Comm: fpu_denormal Not tainted 5.8.0-rc2-gcc-8.2.0-00092-g4ec7aaab0828 #192
NIP: c0000000000100ec LR: c00000000001b85c CTR: 0000000000000000
REGS: c000001dd818f770 TRAP: 1500 Not tainted (5.8.0-rc2-gcc-8.2.0-00092-g4ec7aaab0828)
MSR: 900000000290b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE> CR: 24002884 XER: 20000000
CFAR: c00000000001005c IRQMASK: 1
GPR00: c00000000001c4c8 c000001dd818fa00 c00000000171c200 c000001dd8101570
GPR04: 0000000000000000 c000001dd818fe90 c000001dd8101590 000000000000001d
GPR08: 0000000000000010 0000000000002000 c000001dd818fe90 fffffffffc48ac60
GPR12: 0000000000002200 c000001ffff4f480 0000000000000000 0000000000000000
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR20: 0000000000000000 00007fffab225b40 0000000000000001 c000000001757168
GPR24: c000001dd8101570 c0000018027b00f0 c000001dd8101570 c000000001496098
GPR28: c00000000174ad05 c000001dd8100000 c000001dd8100000 c000001dd8100000
NIP save_fpu+0xa8/0x2ac
LR __giveup_fpu+0x2c/0xd0
Call Trace:
0xc000001dd818fa80 (unreliable)
giveup_all+0x118/0x120
__switch_to+0x124/0x6c0
__schedule+0x390/0xaf0
do_task_dead+0x70/0x80
do_exit+0x8fc/0xe10
do_group_exit+0x64/0xd0
sys_exit_group+0x24/0x30
system_call_exception+0x164/0x270
system_call_common+0xf0/0x278

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Split out of fix patch, add oops log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200708074942.1713396-1-npiggin@gmail.com