arch/arm/cmake: Do not link an executable to detect the compiler.

arch/arm/src/cmake/Toolchain.cmake

@@ -25,6 +25,23 @@
 set(CMAKE_SYSTEM_NAME Generic)
 set(CMAKE_SYSTEM_VERSION 1)
 
+# Compiler detection must not try to link a full executable.
+#
+# CMake validates a compiler by building and linking a test program.  For a bare
+# metal cross toolchain that link cannot succeed on its own terms: it has no
+# start files, no default linker script and no libc, and the flags NuttX
+# supplies for it (CMAKE_EXE_LINKER_FLAGS_INIT below adds --specs=nosys.specs)
+# assume a toolchain shipped with newlib.  A perfectly usable arm-none-eabi-gcc
+# without those specs then fails configuration before a single NuttX source file
+# is considered:
+#
+# arm-none-eabi-gcc: fatal error: cannot read spec file 'nosys.specs'
+#
+# Building a static library instead exercises the compiler and stops short of
+# the link, which is the documented approach for this case.
+
+set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
+
 set(ARCH_SUBDIR)
 
 if(CONFIG_ARCH_ARMV7A) # ARMv7-A