libc: arm: Implement setjmp GCC backwards compatibility.

When compiling Newlib for arm targets with GCC 12.1 onward, the
passing of architecture extension information to the assembler is
automatic, making the use of .fpu and .arch_extension directives
in assembly files redundant.

With older versions of GCC, however, these directives must be
hard-coded into the `arm/setjmp.S' file to allow the assembly of
instructions concerning the storage and subsequent reloading of the
floating point registers to/from the jump buffer, respectively.

This patch conditionally adds the `.fpu vfpxd' and `.arch_extension
mve' directives based on compile-time preprocessor macros concerning
GCC version and target architectural features, such that both the
assembly and linking of setjmp.S succeeds for older versions of
Newlib.

newlib/libc/machine/arm/setjmp.S

@@ -64,6 +64,28 @@
 
 	.syntax unified
 
+/*  GCC 12.1 and later will tell the assembler exactly which floating
+    point (or MVE) unit is required and we don't want to override
+    that.  Conversely, older versions of the compiler don't pass this
+    information so we need to enable the VFP version that is most
+    appropriate.  The choice here should support all suitable VFP
+    versions that the older toolchains can handle.  */
+#if __GNUC__ && __GNUC__ < 12
+/*  Ensure that FPU instructions are correctly compiled and, likewise,
+    the appropriate build attributes are added to the resulting object
+    file.  Check whether the MVE extension is present and whether
+    we have support for hardware floating point-operations.  VFPxd
+    covers all the cases we need in this file for hardware
+    floating-point and should be compatible with all required FPUs
+    that we need to support.  */
+# if __ARM_FP
+	.fpu vfpxd
+# endif
+# if __ARM_FEATURE_MVE
+	.arch_extension mve
+# endif
+#endif
+
 #if __ARM_ARCH_ISA_THUMB == 1 && !__ARM_ARCH_ISA_ARM
 /* ARMv6-M-like has to be implemented in Thumb mode.  */