testing/sig_sp_test: add test for SP context restore on signal return

[cshung]

Summary

Add a test application that verifies modifying REG_R13 (SP) in saved register context during a signal handler is honored on exception return.

Test scenario (simulates a managed runtime unwinding past a trampoline):

1. Push values 1 and 2 onto the stack (simulating trampoline pushes)
2. Wait for SIGALRM (async delivery from timer interrupt)
3. Handler advances SP by 4 (emulates "pop" / unwind past one frame)
4. Handler redirects PC to a resume function
5. Resume function pops one value — verifies it equals 1 (not 2)

This exercises the HW exception frame relocation fix in arm_exception.S.

Dependencies:

• Requires CONFIG_ARMV7M_SP_CONTEXT_RESTORE=y or CONFIG_ARMV8M_SP_CONTEXT_RESTORE=y
• Flat build only (BUILD_FLAT) — accesses nxsched_self() for saved_regs
• ARM architecture only

Companion OS fix PR: apache/nuttx#19123

Impact

• New test application only, no impact on existing code
• Gated by Kconfig dependencies (will not appear unless SP context restore is enabled)

Testing

Host: Ubuntu 22.04 x86_64, arm-none-eabi-gcc 13.3, QEMU 8.2.2 (via Docker)

Targets tested:

• lm3s6965-ek:qemu-flat (ARMv7-M, Cortex-M3)
• mps2-an521:nsh (ARMv8-M, Cortex-M33)

Test log (ARMv7-M):

nsh> sig_sp_test
sig_sp_test: Signal SP restore test
sig_sp_test: push 1, push 2, alarm, handler SP+=4, pop => 1
sig_sp_test: handler - PC=0x00022770 SP=0x2000a9a8
sig_sp_test: handler - new SP=0x2000a9ac PC=0x000226ac
sig_sp_test: popped value = 1 (expected 1)
sig_sp_test: PASS

Test log (ARMv8-M):

nsh> sig_sp_test
sig_sp_test: Signal SP restore test
sig_sp_test: push 1, push 2, alarm, handler SP+=4, pop => 1
sig_sp_test: handler - PC=0x1002b2b8 SP=0x38007b08
sig_sp_test: handler - new SP=0x38007b0c PC=0x1002b1fc
sig_sp_test: popped value = 1 (expected 1)
sig_sp_test: PASS

Sanity check (without SP adjustment in handler):

sig_sp_test: popped value = 2 (expected 1)
sig_sp_test: FAIL - expected 1, got 2