arch_atomic : Introduce CONFIG_LIBC_ARCH_ATOMIC

Some armv7-m-based SoCs do not work with atomic instructions,
even though armv7-m supports them.

To avoid using atomic instructions generated by gcc,
CONFIG_LIBC_ARCH_ATOMIC is newly introduced with which
arch_atomic.c is linked explicitly.

However, the function names need to be changed to avoid
build errors, since the functions described in stdatomic.h
are gcc built-in and inlined when the code is compiled.

About libcxx with CONFIG_LIBC_ARCH_ATOMIC, it still
does not work. It is also needed to call nx_atomic_ ver
instead of __atomic ver in
libcxx/include/__atomic/cxx_atomic_lmpl.h.

Signed-off-by: Takuya Miyasita <Takuya.Miyashita@sony.com>

libs/libc/machine/Kconfig

@@ -9,6 +9,14 @@
 
 menu "Architecture-Specific Support"
 
+config LIBC_ARCH_ATOMIC
+	bool "arch_atomic"
+	default n
+	---help---
+		If this configuration is selected and <include/nuttx/atomic.h> is
+		included, arch_atomic.c will be linked instead of built-in
+		atomic function.
+
 config ARCH_LOWPUTC
 	bool "Low-level console output"
 	default y