toolchain/Config.in: update BR2_TOOLCHAIN_HAS_LIBATOMIC dependencies

Update BR2_TOOLCHAIN_HAS_LIBATOMIC dependencies to avoid the following
build failure with libopenssl and
BR2_TOOLCHAIN_EXTERNAL_BOOTLIN_ARMV7M_UCLIBC_STABLE:

/home/buildroot/autobuild/run/instance-2/output-1/host/opt/ext-toolchain/arm-buildroot-uclinux-uclibcgnueabi/bin/ld.real: ./libcrypto.a(libcrypto-lib-threads_pthread.o): in function `CRYPTO_atomic_or':
threads_pthread.c:(.text+0xfa): undefined reference to `__atomic_is_lock_free'

libatomic is available since gcc 4.8, when thread support is enabled.
However, the gcc logic in libatomic/configure.tgt does not recognize
"uclinux" as a valid OS part of the target tuple, and therefore it
does not build libatomic. The "uclinux" part of the tuple is used by
Buildroot when BR2_BINFMT_FLAT=y [1]. This broken logic has only been
fixed for arm since gcc 10.1.0 [2].

Indeed, bootlin armv7m is an uclibc toolchain compiled with atomic
support through libatomic.

[1] https://git.buildroot.net/buildroot/commit/?id=b3d1fb26dcadd8c570e2c415ce05398ecc810b32
[2] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=b1e21e5a5d19b436f948710e09157c5b3244f541

Fixes:
 - http://autobuild.buildroot.org/results/d25e898f9715bf6a21284807361a57735a7a2e1d
 - http://autobuild.buildroot.org/results/e37ed5ad6ba41d610bffe9c234f699e203ef5069

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 8cb0f6e011d9975f7c4f8b2dc553b086bceef1dc)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

toolchain/Config.in

@@ -879,15 +879,19 @@ config BR2_TOOLCHAIN_HAS_SYNC_8
 	default y if BR2_TOOLCHAIN_ARM_HAS_SYNC_8
 	default y if BR2_TOOLCHAIN_X86_HAS_SYNC_8
 
-# libatomic is available since gcc 4.8, when thread support is
-# enabled. Also, libatomic doesn't recognize "uclinux" as a valid OS
-# part of the tuple, and is therefore not build on uclinux targets,
-# which is why BR2_BINFMT_FLAT configurations are excluded.
+# libatomic is available since gcc 4.8, when thread support is enabled.
+# However, the gcc logic in libatomic/configure.tgt does not recognize
+# "uclinux" as a valid OS part of the target tuple, and therefore it
+# does not build libatomic. The "uclinux" part of the tuple is used by
+# Buildroot when BR2_BINFMT_FLAT=y. This broken logic has only been
+# fixed for arm since gcc 10.1.0.
 config BR2_TOOLCHAIN_HAS_LIBATOMIC
 	bool
-	default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_8 && \
-		BR2_TOOLCHAIN_HAS_THREADS && \
-		!BR2_BINFMT_FLAT
+	default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
+	depends on BR2_TOOLCHAIN_HAS_THREADS
+	depends on BR2_BINFMT_ELF || \
+		(BR2_BINFMT_FLAT && BR2_arm && \
+		BR2_TOOLCHAIN_GCC_AT_LEAST_10)
 
 # __atomic intrinsics are available:
 # - with gcc 4.8, either through built-ins or libatomic, on all