testing/ostest: build the perf test only where it can link

[casaroli]

Summary

testing/ostest cannot link in a protected or kernel build whenever
CONFIG_ARCH_PERF_EVENTS is set.

perf_gettime() and perf_getfreq() are kernel functions, defined in
sched/clock/clock_perf.c. Neither appears in syscall.csv, and
libs/libc/sched/clock_perf.c supplies only perf_gettime(), gated on
CONFIG_ARCH_HAVE_PERF_EVENTS_USER_ACCESS. So user space has no way to reach
either symbol unless the build is flat.

The pieces disagree about when to use them:

• testing/ostest/Makefile / CMakeLists.txt compile perf_gettime.c when
  CONFIG_ARCH_HAVE_PERF_EVENTS is set.
• ostest_main.c calls perf_gettime_test() when CONFIG_ARCH_PERF_EVENTS
  is set and CONFIG_ARCH_PERF_EVENTS_USER_ACCESS is not — which is exactly
  the case where the symbols are kernel-only.

So any protected/kernel configuration with ARCH_PERF_EVENTS=y fails to link:

ostest/perf_gettime.c:91:  undefined reference to `perf_gettime'
ostest/perf_gettime.c:184: undefined reference to `perf_getfreq'
make[1]: *** [nuttx_user.elf] Error 1

mps3-an547:knsh is one, because its defconfig sets CONFIG_SCHED_IRQMONITOR,
which makes ARCH_PERF_EVENTS default y. mps2-an521:knsh and
mps2-an500:knsh escape only because they set neither SCHED_IRQMONITOR nor
SCHED_CRITMONITOR, so ARCH_PERF_EVENTS is off, the call compiles out and
the archive member is never pulled in.

This adds CONFIG_BUILD_FLAT to all four sites, which is where ostest links
against the kernel's own copy of these functions.

Impact

No configuration that runs this test today stops running it — flat builds are
untouched. Protected and kernel builds that previously could not link ostest
at all now build, and skip the one test they could never have executed.

Users affected: anyone enabling TESTING_OSTEST on a BUILD_PROTECTED or
BUILD_KERNEL target with SCHED_IRQMONITOR, SCHED_CRITMONITOR,
RPMSG_PING or SEGGER_SYSVIEW enabled (the four that make
ARCH_PERF_EVENTS default y).

No API, Kconfig or documentation change.

Testing

Host: macOS 15.5 (Darwin 25.5.0), Apple Silicon; arm-none-eabi-gcc 14.2.Rel1;
QEMU 11.0.3.

Target: mps3-an547:knsh — Armv8-M, Cortex-M55, BUILD_PROTECTED, the
configuration that exposed this.

Before — link fails:

arm-none-eabi-ld: apps/testing/ostest/perf_gettime.c:91:(.text.perf_gettime_test+0x12):
    undefined reference to `perf_gettime'
arm-none-eabi-ld: apps/testing/ostest/perf_gettime.c:184:(.text.perf_gettime_test+0x1bc):
    undefined reference to `perf_getfreq'
make[1]: *** [nuttx_user.elf] Error 1
make: *** [nuttx] Error 2

After — both blobs link:

   text	   data	    bss	    dec	    hex	filename
 256663	   1020	  10160	 267843	  41643	nuttx
 176857	    640	   3088	 180585	  2c169	nuttx_user.elf

and the image boots to NSH and runs ostest under
qemu-system-arm -M mps3-an547.

The flat path is unchanged by construction — the patch only ever adds a
condition, so no build that compiled perf_gettime.c before and still
satisfies CONFIG_BUILD_FLAT is affected.