[PATCH] libc: arm: Implement setjmp GCC backwards compatibility.

[Victor L. Do Nascimento]

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' directives instructions in assembly files
redundant.

With older versions of GCC, however, the .fpu directive 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' directive 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 | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/newlib/libc/machine/arm/setjmp.S b/newlib/libc/machine/arm/setjmp.S
index c615f2428..3a9aa840d 100644
--- a/newlib/libc/machine/arm/setjmp.S
+++ b/newlib/libc/machine/arm/setjmp.S
@@ -64,6 +64,22 @@
 
 	.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
+/*  While GCC > 10.1 supports MVE, the MVE instructions do not need an
+    .fpu directive, so we don't need to handle that case.  VFPxd thus
+    covers all the cases we need in this file and should be compatible
+    with all required FPUs that we need to support.  */
+# if __ARM_FP
+	.fpu vfpxd
+# endif
+#endif
+
 #if __ARM_ARCH_ISA_THUMB == 1 && !__ARM_ARCH_ISA_ARM
 /* ARMv6-M-like has to be implemented in Thumb mode.  */
 
-- 
2.36.1